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tinct, and, moreover, widely separated from each other by intervention of a special ossicle, doubtless a sesamoid, in the axis of the foot immediately above the single terminal phalanx.

The actual structure, both of the bones of the digits and of the horny hoof, will be appreciated from a glance at the accompanying figure. This is engraved of life size, front view, with the hoof withdrawn sufficiently to display all the parts. The preparation is from a young subject about three months old, in which the proximal epiphyses of the phalanges are still evident. The pair of distinct proximal phalanges of normal characters, or nearly so, are seen to be succeeded by nodular medial phalanges, which latter, as well as the distal extremities of the proximal phalanges, are widely separated by intervention of a special ossicle in the axis of the foot. To these succeeds a single broad and flattened terminal phalanx, obviously composed of the pair of distal phalanges anchylosed together. In this specimen, the ancbylosis is complete, even at so early an age of the subject; its condition apparently being not the result of progressive confluence of the two bones, but of their original connation.

The terminal phalanx is flattened and somewhat scooped out on its posterior aspect, without trace of previous separation into halves. In front, however, as shown by the figure, it presents a central triangular elevation, apex downward, and base articulated with the nodular ossicle above it, as if a wedge of bone had been thrust into the axis of the limb between the primitive distal phalanges. This wedgeshaped piece of bone is completely auchylosed with the present single distal phalanx; and below its apex the edge of the bone is perfectly continuous across the axis of the foot.

The central nodular ossicle, which I have already mentioned as a sesa. moid, articulates with all five of the bones of the foot. I cannot account for its presence unless it be a displaced sesamoid, such as for example that which is normal beneath the base of the distal phalanx of the horse, and known to some as the "os subarticulatum". In the normal pig's foot, there are several pairs of sesamoids beneath the phalangeal articulations; and the bone in question may be regarded as a confluence of the pair at the base of the distal phalanges, or of two pairs at the bases of the medial and distal phalanges respectively. The displacement of these sesamoids brings the ossicle into position in the axis of the foot between instead of under the bones. Or, it may be that this ossicle is a confluent pair of sesamoids from beneath the basis of the medial phalanges, and that the wedge-shaped piece of bone which appears upon the front of the distal phalanx, consolidated therewith, represents sesamoids from beneath the distal phalanges.

The horny hoof encases these bones as far as the distal extremities

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of the proximal phalanges. It is perfectly whole, or "solid", as seen in the figure. In front, there is a slight, though evident, vertical line of impression along the middle, indicating its composition from lateral halves. On the sole of the hoof, there is a broad, angular elevation of horny substance, apex forward, and sides running backward and outward to the lateral borders of the hoof, the whole structure being curiously like the frog of the horse's hoof. In fact, it is a frog, though broad, flattened, and somewhat horseshoe-shaped, instead of being narrow, deep,. and acute, as in the actual frog of the horse. This arcuate thickening of the corneous substance occupies about the middle third of the whole plantar surface of the hoof.

Viewing the apparent establishment of this pseudo-perissodactyle structure in an artiodactyle, the question arises whether we have not, under our eyes, an example of a way in which a solidungulate may be evolved from a pluridigitate stock-though of course the one case is by enlargement of a single median digit and reduction of lateral digits, while in the present instance a bone in the axis of the limb is produced by failure of fission between lateral paired digits. Nothing is more certain than that the present solid-hoofed horse has come by direct descent, with modification, from its several-toed ancestors of the Tertiary. In the present case, we seem to have the initial steps of an actual transformation which may in time result in modifications to which ordinal value may attach. It may be suggested that this modification is one of progressive adaptation of the animals to their freely-ranging state on the prairies of the country, just as the series of modifications which the primitive horse's foot has undergone in adaptation to the making of the most serviceable hoof for running on hard ground at the expense of any other function.

ART. XIV.-PROFESSOR OWEN ON THE PYTHONOMORPHA.

BY E. D. COPE.

The British Museum has recently obtained the Van Breda collection of fossils, which includes a valuable series of Mosasauroid remains from Maestricht, the locality which furnished to Cuvier the typical specimen of the Mosasaurus giganteus. Professor Owen has improved the opportunity to study this material with that already in possession of the museum, some of which was derived from North American sources.

In pursuing this subject, Professor Owen has done me the honor to study my contributions to it, a summary of which appears in the second volume of the Final Report of the United States Geological Survey of the Territories under Dr. F. V. Hayden. He follows my determinations and conclusions, and criticises them in the light of his long experience. As a portion of this criticism is adverse to what he supposes my conclusions to be, I propose on the present occasion to give such a brief review of Professor Owen's paper* as my other immediate occupations will permit. I premise that this cannot now include a complete review of the subject, nor the exposition of several parts of it which have not yet received the attention of Professor Owen or of any one else.

Professor Owen's references to my work may be included under three heads, viz:-First, as to matters of fact or observation; second, as to determination of homologies of parts; third, as to the estimation of affinities as derived from the preceding branches of the subject. I now consider

I. QUESTIONS OF FACT.

The many observations as to the structure of the order of Pythonomorpha recorded by me in the volume already referred to are confirmed by Professor Owen with a single exception. He correctly describes the vertebræ of the genus Mosasaurus as without the zygantrum and zygo. sphene articulation, and proceeds to say (p. 709), in reference to my ascription of this structure to the genus Clidastes, that the structure of Mosasaurus "is repeated in plates xviii, xix, xx, xxi, xxiii, xxiv, xxvi, xxvii, xxix, xxx, xxxiv, and xxxv of Professor Cope's great work; in every figure the zygosphene and zy gantrum are absent." And again,—“In the plates xviii and xxiii given to the vertebræ of the species [Clidastes] stenops and planifrons, the parts and processes are as usual not indicated." All this is a remarkable oversight on the part of Professor Owen. He *Quarterly Journal of Geol gical Society, London, 1877, p. 682.

will find the zygosphene distinctly represented on figs. 5 a and 5 b, pl. xviii; figs. 3 b, 3 d, 6 b, 6 c, pl. xix ; fig. 15 d, pl. xxi; figs. 3 c and 3 d, pl. xxiii; fig. 4, pl. xxiv; and the zygantrum in nearly as many figures. He will also find them well represented in the figures of vertebræ of Clidastes on plates v and xii of the Extinct Batrachia and Reptilia of North America. In order to substantiate his position, he copies from my work a figure of a vertebra of Clidastes stenops from which the zygosphene has been accidentally broken away.

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Professor Owen places me in the attitude of committing error in questions of fact in regard to the limb-bones and their arches in the Lacertilia and Ophidia. My statement is," As there are many Lacertilia without limbs, and some serpents with them, their presence in this order is irrelevant in this connection, especially as the arches supporting them are most like those of tortoises and Plesiosaurs." Professor Owen then proceeds to state that there are only twenty-three genera of Lacertilia with reduced limbs, and "extremely few" where they may be considered to be rudiments. Professor Owen can hardly have had in mind the developments of herpetology during the last five or ten years in making this assertion; for the genera of lizards now known in which the limbs are rudimental may safely be said to be numerous, and those with. out even rudiments are not a few. Professor Owen appears to have overlooked the entire suborder of the Amphisbania, which are all limbless with the exception of one genus. He then criticises my reference to serpents with limbs, and observes:-"In certain Ophidia dissection has revealed a small styliform bone on each side the cloaca; in a few it is tipped with horn in the shape of a claw. . . Whether these appendages to the generative parts be homologous with the 'claspers' of sharks or with the ventral fins, and, if the latter, with the hind limbs of lizards, is yet an open question." Reference to the numerous genera and species of serpents which possess rudimental hind limbs, as well as to the two suborders which possess a pelvis, is here entirely omitted, and the demonstration of the homology of the anal claws above mentioned with true hind limbs appears to be unknown to Professor Owen. Besides the Boida, Pythonida, and Xenopeltidæ known to Professor Owen as possessing these rudimental limbs, there are the Lichanurida, Tortricidæ, and Stenostomida; while the Typhlopida and Stenostomida possess a pelvis-the latter family with ilium, ischium, and pubis, as ascertained by Peters. This pelvis is more complete than that of various Lacertilian genera of the Diploglossa group, or of the suborder of the Amphisbania, which consists, according to Stannius, of a rudimental ilium only. My statements on this point are borne out by the facts. My assertion as to the resemblance of the scapular and pelvic arches to those of tortoises and Plesiosaurs is true in view of the fact that the former has no inferior connection with a sternum, so far as known, an element absent in the orders named and the Ophidia, but present in the lizards, although not universally so.

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