has four digits, of which the two outer are small, while the middle ones are large, with immensely powerful claws.

Extinct Types.-The only fossil forms which can be referred to the section of the Insectivora with tritubercular molars are the Leptictide, of the Eocene and Miocene of North America. This family includes the genera Leptictis, Mesodectes, and Ictops, all of which are regarded by Dr. Schlosser as true Insectivora, although they were placed by Professor Cope with the Creodont Carnivora.

Bibliography of Insectivora.-Peters, Reise nach Mossambique-Säugeth. 1852; Id. "Ueber die Classification der Insectivora," Monatsb. Akad. Wissensch. Berlin, 1865, and other papers; Mivart, "On the Osteology of Insectivora,” Journ. Anat. and Phys. 1867, 1868, and Proc. Zool. Soc. 1871; Gill, "Synopsis of Insectivorous Mammals," Bull. Geol. and Geog. Survey, U.S.A. Washington, 1875 (includes a general bibliography of the order); Dobson, Monograph of the Insectivora, Systematic and Anatomical, London, 1882-90.

CHAPTER XIII

THE ORDER CHIROPTERA

MAMMALS, having their fore limbs specially modified for flight. The forearm consists of a rudimentary ulna, and a long curved radius. The carpus has six bones supporting a small pollex and four greatly elongated fingers, between which and the sides of the body and the hinder extremities a thin expansion of the integument (the wing-membrane or patagium) is extended. The knee is directed backwards, owing to the rotation of the hind limb outwards by the wing-membrane; a peculiar elongated cartilaginous process (the calcar), rarely rudimentary or absent, arising from the inner side of the ankle-joint, is directed inwards, and supports part of the posterior margin of an accessory membrane of flight, extending from the tail or posterior extremity of the body to the hinder limbs (the inter-femoral membrane). The penis is pendent; the testes are abdominal or inguinal; the mammary glands thoracic and generally postaxillary; the uterus is simple or with more or less long cornua; the placenta discoidal and deciduate; and the smooth cerebral hemispheres do not extend backwards over the cerebellum. The dental series includes incisors, canines, premolars, and molars and never exceeds i, c1, p 3, m; total 38.

The animals comprised in this order are at once distinguished by the presence of true wings, and this peculiarity is accompanied by other modifications of bodily structure having special relation to flight. Thus, in contrast to most other mammals, in which the hind limbs greatly preponderate in size over the fore, in the present order the fore limbs immensely exceed the short and weak hinder extremities. The thorax, as giving origin to the great muscles which sustain flight, and containing the proportionately large lungs and heart, is remarkably capacious, and the ribs are flattened and close together; the shoulder-girdle is also greatly developed in comparison with the weak pelvic bones.

Linnæus included the Bats among the Primates, mainly on

account of the number of their upper incisors, supposed to be always four, the thoracic position of the mamma, and the pendent condition of the penis. Many other zoologists, taking into consideration the placental characters and the form of the uterus, have followed him; but it is evident that the situation of the mammæ is related to the necessarily central position of the young during flight, the shortness of the uterine cornua, observable in so many species, to the generally uniparous gestation requiring less room, while the discoidal deciduate placenta is equally present in and characteristic of the Insectivora, many species of which also have the penis pendent. Thus, the reasons for maintaining the Bats in this high position being disposed of, we find in the low organisation of their brain a proof of their inferior status; while furthermore, although they differ widely from all other mammals in external form, it is evident that this is only the result of special adaptation to aerial locomotion; and, taking into account their whole bodily structure, we may accept the view of Professor Huxley that they should merely be regarded as exceedingly modified Insectivora.

So thoroughly, however, has this adaptation for flight been carried out that of all animals the Bats are the least terrestrial, not one of them being equally well fitted for progression on the earth. This is due to the hind as well as the fore limbs being pressed into the service of aerial locomotion. Thus the hind limb is so rotated outwards by the wing-membrane that, contrary to what obtains in all other vertebrates, the knee is directed backwards, and corresponds in position to its serial homologue the elbow. It necessarily follows from this arrangement that when a Bat is on the ground it rests on all fours, having the knees directed upwards; while, in order to bring it into a position for forward progression, the foot rotates forwards and inwards on the ankle. Walking under these circumstances is at best only a kind of shuffle, and that this is fully recognised by the animal is evidenced by its great anxiety to take wing, or, if this be impracticable, to ascend to some point where it can hitch itself up by the claws of the hind legs in its usual position when at rest.

The bones of the skeleton are characterised by their slenderness and the great size of the medullary canals in those of the extremities. The vertebral column is short, and the vertebræ differ very slightly in number and form throughout the species. The general number of the dorso-lumbar vertebræ is 17, of which 12 are dorsal; the cervicals are very broad, but short from before backwards, their breadth being due to the great transverse diameter of the spinal canal rendered necessary by the comparatively large size of the spinal cord, which, after giving off the nerves to the fore limbs and thorax, rapidly diminishes in size, and in the lumbo-sacral region is reduced to a fine thread. Except in the

frugivorous Pteropodide, the vertebræ, from the third cervical backwards, are devoid of neural spines. From the first dorsal to the last lumbar vertebra the spinal column forms a single curve backwards, which is most pronounced in the lumbar region. The centra of the vertebræ are but slightly movable upon each other, and in old individuals appear to become partially ankylosed together. The caudal vertebræ are simple cylindrical bones without processes; their number and length being extremely variable even in closely allied species; and the anterior caudals are generally united to the

FIG. 297.-Skeleton and flying-membranes of the Noctule Bat (Vesperugo noctula). x . c, Clavicle; h, humerus; r, radius; u, ulna (rudimentary); d1, pollex; d2, d3, d4, d5, other digits of the manus supporting wm, the wing-membrane; m, m, metacarpal bones; ph1, first phalanx; ph2, second phalanx; ph3, third phalanx; am, antebrachial membrane; f, femur; t, tibia; fb, fibula (rudimentary); c, calcar supporting im, the interfemoral membrane; pcl, postcalcaneal lobe.

ischial tuberosities. The relative development of the caudal vertebræ is, indeed, intimately correlated to the habits of the animals; the long tail in the insectivorous forms supporting and controlling the position of the large interfemoral membrane, which appears not only to aid their rapid motions when in pursuit of their prey by acting as a rudder, but also to assist in the capture and retention of the larger insects. In the frugivorous types, on the other hand, this is not required, and the tail is accordingly rudimentary or absent. In all Bats the presternum has a prominent keel for the attachment of the great pectoral muscles. In most

species the ribs are much flattened, and in some they are partially ankylosed by their contiguous margins.

The skull is subject to considerable structural variations, even within the limits of a single family. Postorbital processes to the frontals are found only in the Pteropodide and some Nycteride and Emballonuride. Pteropus leucopterus and Pteraloper are peculiar in having the orbit completely surrounded by bone. A slender zygomatic arch is present, except in some of the Phyllostomatida.

The milk-teeth are peculiar in that they are utterly unlike those of the permanent series. They are slender, with sharp recurved cusps; and as a rule are shed at an early period (in the Rhino lophida before birth), but may coexist with some of the fully developed permanent teeth. The permanent teeth are subject to great variation of form, although they always have distinct roots. In the Insectivorous types they are acutely cusped, the cusps in those of the upper jaw being arranged in a more or less distinct W; but in the frugivorous forms, like the Pteropodide and some of the Phyllostomatida, the molars are longitudinally grooved or hollowed

out.

The pectoral girdle maintains a very constant type. Thus the clavicle is very long, strong, and curved; and the scapula large, oval, triangular, with a long curved coracoid process. The humerus, though long, is scarcely two-thirds the length of the radius. The ulna is rudimentary, its proximal extremity, which articulates with but a small part of the humerus, being ankylosed to the radius ; and immediately beyond the joint it is reduced to a slender splintlike bone, extending about as far as the middle of the radius. In all species a detached sesamoid bone exists in the tendon of the triceps muscle. The radius is very long, in some species actually equal to the length of the head and body. The proximal row of the carpus consists of a single bone formed by the united scaphoid, lunar, and cuneiform; which, with the extremity of the radius, forms the radio-carpal joint. In the distal row the trapezium, trapezoid, and magnum vary in size in the different families, the unciform appearing to be the most constant, and the pisiform being generally very small.

The manus is always furnished with five digits. The first, fourth, and fifth digits consist of a metacarpal and two phalanges; but in the second and third digits the number of phalanges is different in certain families. The pollex always terminates in a claw, which-like the proximal phalanx-is best developed in the frugivorous species. In most of the frugivorous Pteropodide the second digit is provided with a claw; but in all other Bats this and the remaining digits are unarmed. In the genus Trianops alone a very peculiar short bony process projects from the outer side of