Page images
PDF
EPUB

same structure wbich is shown by a like section of a tooth of the Lepidosleus oxyurus.* The same principle of dental composition is exemplified in the teeth of most of the ganoid fishes of the Carboniferous and Devonian systems, and is carried out to a great and bcantiful degree of complication in the old red Dendro. donts. The repetition of the same principle of dental structure in one of the earliest genera of Reptilia, associated with the defect of ossification of the endoskeleton and the excess of ossification in the exa-skeleton of the head, decisively illustrate the true affinities and low position in the Reptiliau class of the so-called Archegosauri. Por other details of the peculiar and interesting structure of the animals representing the earliest or oldest known order of Reptiles, Prof. Owen referred to the article “ Palæontology" in the “Encyclopædia Britannica.' This order is "carboniferous."

Order II.Labyrinthodontia.—Head defended, as in the Ganocephala, by a continuous casque of externally eculptured and unusually hard and polished osseous plates, including the supplementary “postorbital” and “supertemporal” bones, but leaving a “foramen parietale.”+ Two occipital condyles. Vomer divided and dentigerous. Two nostrils. Vertebral centra, as well as arches, ossified, biconcave. Pleurapophyses of the trunk, long and bent. Teeth rendered complex by undulation and side branches of the converging folds of cement, whence the name of the order. Osseous scutes in some. The reptiles presenting the above charac. ters have been divided, according to minor modifications exemplified by the form and proportions of the skull, by the relative position and size of orbital, nasal and temporal cavities, &c., into the several genera ; as e.g. Mastodonsaurus. Trematosaurus, Metopias, Capitosaurus, Zygosaurus, Xestorrhytias. The relation of these remarkable reptiles to the Saurian order has been advocated as being one of close and true affinity, chiefly on the character of the extent of ossification of the skull and of the outward sculpturing of the cranial bones. But the true nature of some of these bones appears to have been overlooked, and the gaze of research for analogous structures has been too exclusively upward. If directed downward from the Labyrinthodontia to the Ganocephali, and to certain gavoid fishes, it suggests other conclusions, which had been worked out by Prof. Owen, in his article on “ Palæontology,” above referred to. There is nothing in the known structure of the so-pamed Archegosaurus or Mastodonsaurus that truly indicates a belonging to the Saurian or Crocodilian-order of reptiles. The exterior ossifications of the skull and the capine-shaped labyrinthic teeth are both examples of the Salamandroid modification of the ganoid type of fishes. The small proportion of the forelimb of the Mystriosaurus in nowise illustrates this alleged saurian affinity; for though it be as short as in Archegosaurus, it is as perfectly constructed as in the Crocodile, whereas the short fore-limb of Archegosaurus is constructed after the simple type of that of the Proteus and Siren. But the futility of this argument of the sauroid affinities is made manifest by the proportions of the bind-limb of Archegosaurus ; it is as stunted as the fore-limb. In the Labyrinthodonts it presented larger proportions, which, however, may be illustrated as naturally by these proportions of the limbs in certain Batrachia, as in the Teleosaurus.

• Wyman,' American Journal of the Natural Sciences,' October, 1843. + The corresponding vacuity is larger in some ganoid fishes.

Order III.-Ichthyopterygia.—The bones of the head still include the supplementary " post-orbitals” and “supra-temporals,” but there are sınall tempural and other vacuities between the cranial bones : a “foramen parietale,” a single convex occipital condyle,* and one comer which is edentulous. Two "antorbital nostrils. Vertebral centra, ossified, biconcave. Pleurapophyses of the trunk long and bent the anterior ones with bifurcate heads. Teeth with converging folds of cement at their base ; implanted in a common alveolar groove, and confided to the maxillary, premaxillary, and premandibular bones. Premaxillaries much exceeding the maxillaries in size. Orbit very large : a circle of sclerotic plates. Limbs natatory; with more than five multi-articulate digits ; no sacrum. With the retention of characters which indicate, as in the preceding orders, an affinity to the higher Ganoiden, the present exclusively marine Reptilia more directly exemplify the Ichthyic type in the proportions of the premaxillary and maxillary bones ; in the shortness and great number of the biconcave vertebræ; in the length of the pleur. apophyses of the vertebræ near the head; in the large proportional size of the eyeball, and its well-ossified sclerotic coat, and especially in the structure of the pectoral and ventral fins. The skin is naked. The order ranges from the lias to the chalk.

Order IV.–Sauropterygia.-No post-orbital and supra-temporal bone :t large temporal and other vacuities between certain cranial bones; a foramen parietale; two antorbital postrils ; teeth simple, in distinct sockets of premaxillary, maxillary, and premandibular bones, rarely on the palatine or pterygoid bones; maxillaries larger than premaxillaries. Limbs natatory; not more than five digits. A sacrum of one or two vertebræ for the attachment of the pelvic arch in some, numerous cervical vertebræ in most. Pleurapophyses with simple heads ; those of the trunk long and bent. In the Pliosaurus the neck vertebræ are comparatively few in number, short and flat. The sauropterygian type seems to have attained its maximum dimensions in this genus: the species of which are peculiar to the Oxfordian and Kimmeridgian divisions of the Upper Oolitic system. M. von Meyer regards the pumber of cervical vertebræ and the length of neck as characters of prime importance in the classification of Reptilia, and founds thereon his Order called Macrotrachelen, in which he includes Simosaurus, Pistosaurus and Nothosaurus, with Plesiosaurus. No doubt the number of vertebræ in the saine skeleton bears a certain relation to ordinal groups; the Ophidia find a com. mon character therein ; yet it is not their essential character; for the snake-like form, dependent on multiplied vertebræ, characterizes equally certain Batracbians (Cæcilia) and fishes (Muræna). Certain regions of the vertebral column are the seats of great varieties in the same natural group of Reptilia. We have longtailed and short-tailed lizards : but do not, therefore, separate those with numerous eaudal vertebræ, -as " Macrourau," from those with few or none. The extinct Dolichosaurus of the Kentish chalk, with its procælian vertebræ, cannot be ordin. ally separated, by reason of its more numerous cervical vertebræ, from other shorter-necked procælian lizards. As little can we separate the short-necked and, the big-headed amphícælian Pliosaur from the Macrotrachelians of Von Meyer.

This character is retained throughout the rest of the class, save in Batrachia, and will not afterwards be expressed in their characters.

+ These bones do not reappear in the subsequent orders.

with wbich it has its most intimate and true affinities. There is much reason, indeed, to suspect that some of the Muschelkalk Saurians, which are as closely allied to Nothosaurus as Pliosaurus is to Plesiosaurus, may have presented apalogous miudifications in the number and proportions of the cervical vertebræ. It is hardly possible to contemplate the broad and short-snouted skull of the Simosaurus, with its proportionably large teeth, without inferring that such a head must have been supported by a shorter and more powerful neck than that which bore the long and slender head of the Nothosaurus or Pistosaurus. The like inference is more strongly impressed upon the mind by the skull of the Placodus, still shorter and broader than that of Simosaurus, and with vastly larger teeeh, of a shape indicative of their adaptation to crushing molluscous or crustaceous shells. Neither the proportions and armature of the skull of Placodus, nor the mode of obtaining the food indicated by its cranial and dental characters, permit the sup position that the head was supported by other than a comparatively short and strong neck, Yet the composition of the skull, its proportions, cavities and other light giving anatomical characters, all bespeak the close essential relationship of Placodus to Simosaurus and other so-called Macrotrachelian reptiles of the Muschelkalk beds. Prof. Owen continued, therefore, as in his Report of 1841, to regard he fin-like modification of the limbs as a better ordinal character than the number, of vertebræ in any particular regoin of the spine. Yet this limb-character is subordinate to the characters derived from the structure of the skull and of the teeth. If, therefore, the general term Enaliosauria may be sometimes found convenient in its application to the natatory group of Saurian Reptiles, the essential distinctness of the orders Sauropterygii avd Ichthyopterygii, typified by the Ichthyosaurus and Plesiosaurus respectively, should be borne in mind. The Plesiosaurus, with its very numerous cervical vertebræ, sometimes thirty in number may be regarded as the type of the Sauropterygii, or pentadactyle sea-lizards. Of all existing reptiles, the lizards, and, amongst these, the Old World monitors, (Varanus, Fitz.), by reason of the cranial vacuities in front of the orbits, most resemble the Plesiosaur in the structure of the skull; as in the division of the nostrils, the vacuities in the occipital region between the exoccipitals and tympanics, the parietal foramen, the zygomatic extension of the post-frontal, the palatomaxillary, and pterygo-sphenoid vacuities in the bony palate ; and all these are lacertian characters as contradistinguished from crocodilian ones. But the antorbital vacuities, between the nasal, prefrontal, and maxillary bones, are the sole external vostrils in the Plesiosaurs. The zygomatic arch abuts against the fore part of the tympanic and fixes it: a much greater extent of the roof of the mouth is ossified than in lizards, and the palato-maxillary and pterygo-sphenoid fissures are reduced to small size. The teeth, finally, are implanted in distinct sockets. That the Plesiosaur had the “ head of a lizard” is an emphatic mode of expressing the amount of resemblance in their cranial conformation. The crocodilian affinities, however, are not confined to the teeth, but are exemplified in some particulars of the structure of the skull itself. In the simple mode of articulation of the ribs the lacertian affinity is again strongly manifested; but to this vertebral character such affinity is limited. All the others exemplify the ordinal distinction of the Plesiosaurs from known existing reptiles. The shape of the joints of the centra ; the number of vertebræ between the head and tail, especially of those of the neck; the slight indication of the sacral vertebræ ; the non-confluence of the caudal bæmapophyses with each other, are all “ plesiosauroid.” In the size and number of abdominal ribs and sternum may perhaps be discerned a first step in that series of development of the hæmapophyses of the trunk, which reaches its maximum in the plastron of the Chelonia. The connexion of the clavicle with the scapula is common to the Chelonia with the Plesiosauri; the expansion of the coracoidsextreme in Plesiosauri-is greater in Chelonia than in Crocodilia ; but is still greater in some Lacertia. The form and proportions of the pubis and ischium, as compared with the ilium, in the pelvic arch of the Plesiosauri, find their nearest approach in the pelvis of marine Chelonia ; and no other existing reptile now offers so near, although it be so remote, a resemblance to the structure of the paddles of the Plesiosaur. Both Nothosaurus and Pistosaurus had many neck-vertebræ, and the transition from these to the dorsal series was effected, as in Plesiosaurus, by the ascent of the rib-surface from the centrum to the neurapophysis ; but the surface, when divided between the two elements, projected further outwards than in most Plesiosauri. In both Nothosaurus and Pistosaurus the pelvic vertebra developes a combined process (par- and di-apophysis). but of relatively larger, vertically longer size, standing well out, and from near the fore part of the side of the vertebra. This process with the coalesced riblet indicates a stronger ilium, and a firmer base of attachment of the hind limb to the trunk than in Plesiosaurus. Both this structure and the greater length of the bones of the fore-arm and leg show that the Muschelkalk predecessors of the liassic Plesiosauri were better organized for occasional progression on dry land. The Sauropterygii extend from the Trias to the chalk inclusive.

Order V.- A nomodontia (avouos, lawless, odovs, tooth). --This order is represented by three families, all the species of which are extinct, and appear to have been restricted to the triassic period. Teeth wanting, or confluent with tusk-shaped premaxillaries, or confined to a single pair in the upper jaw, which bave the form and proportions of canine tusks. A foramen parietale and two nostrils, tympanic pedicle fixed. Vertebræ biconcave; pleurapophysis of the trunk long and curved the anterior ones with bifurcate heads; a sacrum of four or five vertebræ forming with broad iliac and pubic bones, a large pelvis. Limbs ambulatory. — Family Dicynodontia. A long ever-growing tusk in each maxillary bone ; premaxillaries connate, and forming with the lower jaw a beak-shaped mouth, probably sheathed with horn. This includes two genera--Dicynodon and Ptychognathus--all the known species of which are founded on fossils from rocks of probably triassic age in South Africa. - Family Cryptodontia. Upper as well as lower jaw edeotulous, The genus Oudenodon closely conforms to the dicynodont type, and the species are from the same rocks and localities. ---Family (pathodontia. Two curved tusksbaped bodies holding the place of the premaxillaries, and consisting of confluent, dentinal and osseous substance, descending in front of the symphysis mandibulæ. These bodies are homologous with the pair of confluent premaxillary teeth and bones in the existing New Zealand amphicælian lizard Rhynchocephalus; they are analogous to the tusks in the Dicynodonts, and must have served a similar purpose in the extinct reptiles of the New Red (Trias) Sandstone of Shropshire (Rhynchosaurus), in which alone this structure, with an otherwise edentulous beak-shaped

Vol. V.

G

mouth, has hitherto been met with. To this order belongs the Rynchosauroid reptile, from the Elgin sandstone, with palatal teeth, called Hyperodapedon, by Prof. Huxley.

Order VI. Pterosauria.--Although some members of the preceding Order resembled birds in the shape or the edentulous state of the mouth, the reptiles of the present order make a closer approach to the feathered class in the texture and and pneumatic character of most of the bones, and in the modification of the pectoral limbs for the function of flight. This is due to the elongation of the antebrachial bones, and more especially to the still greater length of the metacarpal and phalangial bones of the fifth or outermost digit, the last phalanx of which terminates in a point. The other fingers were of more ordinary length and size, and were terminated by claws, the number of their phalanges progressively increasing to the fourth, which had four joints. The whole osseous system is modified in accordance with the possession of wings: the bones are light, hollow, most of them permeated by air-cells, with thin, compact outer walls. The scapula and coronoid are long and narrow, but strong. The vertebræ of the neck are few but large and strong, --for the support of a large head with long jawe, armed with sharp-pointed teeth. The skull was lightened by large vacuities, of which one was interposed between the nostril and the orbit. The vertebræ of the back are small 18 are those of the sacrum, which were from two to five in number, but combined with a small pelvis and weak hind-limbs, bespeaking a creature unable to stand and walk like a bird ; the body must have been dragged along the ground like that of a bat. The vertebral bodies were united by ball-and-socket joints, the cup being anterior, and in them we have the earliest manifestation of the “procoelian" type of vertebra. The Pterosauria are distributed into genera according to modifica. tions of the jaws and teeth. In the oldest known species, from the lias, the teeth are of two kinds : a few, at the fore part of the jaws, are long, large, sharp-pointed, with a full elliptical base, in distinct and separated sockets; behind them is a close-set row of short, compressed, very small, lancet-shaped teeth. These form the genus Dimorphodon, Ow. In the genus Ramphorynchus, V. M., the fore par. of each jaw is without teeth, and may have been incased by a horny beak; but behind the edentulous produetion there are four or five large and long teeth fol. lowed by several smaller ones. The tail is long, stiff, and slender. In the genus Pterodactylus, Cuv., the jaws are provided with teeth to their extremities; all the teeth are long, slender, sharp-pointed, set well apart. The tail is very short. P. longirostris, Ok., about ten inches in length. From lithographic slate at Pappenheim, P. crassirostris. Goldf., about one foot long, P. Sedgwickii, Ow., from the greensand, with an expanse of wing of twenty feet, exemplify the Prerodactyles proper. The oldest well-known Pterodactyle is the Dimorphodon macronys of the lower lias; but bones of Pterodactyle have been discovered in coeval lias of Wirtemberg. The next in point of age is the Dimorphodon Banthensis, from the

Posidonomyed-Schiefer” of Banz in Bavaria, answering to the alum shale of the Whitby lias. Then follows the P. Bucklandi, from the Stonesfield oolite. Abore this come the first-defined and numerous species of Pterodactyle from the lithographic slates of the middle oolític system in Germany, and from Oirin on the Rhone. The Pterodactyles of the Wealden are, as yet, known to us by only a few

« EelmineJätka »