dages of the posterior limbs, a higher development than in fishes; beyond the sacrum all the vertebræ are caudal, in which the pleurapophyses become gradually shorter, a few of the first attached to diapophyses, and the hæmapophyses articulated between and to 2 vertebral bodies. In this class we see for the first time regions of the body. In the bird skeleton the premaxillary is much more developed than the maxillary, the reverse of what is seen in reptiles; the greater volume of the brain requires an increased cranial cavity, which is obtained by the expansion of the neural arches and spines without the addition of any new bones; the cervical segments have short and free pleurapophyses or ribs, which are early united to the neural arches, forming numerous simple vertebræ, and giving length and flexibility to the neck; the detached hæmapophyses of the atlas are usually joined together at their extremities, forming a thin osseous arch, the furcular bone; in the thorax the latter are ossified into sternal ribs, the pleurapophyses being the vertebral ribs, bearing diverging appendages, pointing backward, which serve to unite the ribs and to render the thorax more solid; the hæmal spines of the anterior thoracic segments are developed into the broad sternum characteristic of birds, with its keel on the median line large in proportion to the powers of flight. The sacral region is greatly developed, both in the extent and in the number of bones firmly united to form it, and in its enormous pleurapophyses, especially the ilium; the sacrum includes some of the last dorsal, the lumbar, the sacral, and even some of the caudal vertebræ as limited in the reptile skeleton; after the sacrum come 5 or 6 caudal, more or less united, the last compressed laterally and directed upward. The pelvis has only 2 hæmapophyses, the pubis and the ischium, not united on the median line, except in the ostrich for the former and the nandou (rhea) for the latter, the rule being that the pelvis of birds is open below. The diverging appendages of the scapular and pelvic arches, or the anterior and posterior limbs, agree in having only 2 bones in the carpus and tarsus, and 3 united in the metacarpus and metatarsus, supporting in the former the 2d, 3d, and 4th phalanges, that of the 2d very rudimentary; in the metatarsus the 3 bones are, except in the pen guins, united for their whole length, including also the 2 tarsal bones; the rudimentary metatarsal of the great toe is not anchylosed, and is directed backward, supporting the hind toe with 2 phalanges; the 2d toe has 3 phalanges, the 3d 4, the 4th 5, the 5th or little toe being wanting; by the number of phalanges we know that, in the African ostrich, the external or shortest toe of the 2 by its 5 joints is the 4th, and the internal the 3d, longer than the other, though having only 4 joints. In the mammal skeleton the cranial cavity is expanded, as in birds, chiefly at the expense of the neural spines, frontal, parietal, and occipital; but in most the squamous portion of the temporal

bone forms part of the cranial walls; the occipital is articulated to the atlas by zygapophyses or condyles developed from the neural arches, and the hæmal or scapular arch is generally far removed from the skull; the hamapophyses of the atlas, or clavicles, vary much in extent, degree of ossification, and even in their presence; the pleurapophyses of the cervical vertebræ are very short, and are generally united to the other elements, circumscribing the foramen for the vertebral artery. The number of the cervicals is 7, except the alleged 6 in the manatee and the 8 or 9 in the 3-toed sloth; this number depends on the existence of the diaphragm, &c., determining the number and distribution of the pairs of cervical nerves; in some whales and armadillos they are more or less consolidated. In the dorsals the pleurapophyses or ribs are movable, and the anterior ones are articulated between 2 vertebra; the hæmapophyses are the costal cartilages, and the hæmal spines are generally a distinct chain of bones, in the highest consolidated into a sternum; toward the loins the pleurapophyses become shorter, and are attached to their respective vertebral centres and to the diapophyses; the hæmapophyses become shorter, and finally free and floating. The caudals vary much in number, size, and form; the short pleurapophyses in the anterior ones are developed at the end of diapophyses, and the hæmal arch, when it exists, is articulated directly to the bodies. The limbs (except in cetaceans, where the posterior are wanting) are much alike, whether adapted for flying, digging, swimming, running, or climbing, as will be seen under the homotypes below. In the fish and reptile the vertebral column is straight or nearly so; in the bird the skull forms a right angle with the neck, the latter having a sigmoid curve, and the tail bent upward; in the springing mammals, like the carnivora, there is a convergence of the spinous processes toward the 11th dorsal, and in most there is a similar convergence toward the 4th cervical, these two regions being the centres of special movements of the column; in bulky animals, like the elephant, which move with a rigid spine, these processes are all inclined a little backward, as in crocodilians. In man the spine has several slight and graceful curves, destined to prevent shocks to the nervous system from movements incidental to the erect position; the curvature of the sacrum and coccyx are greatest compared with the number of vertebræ, and the anterior and posterior diverging appendages reach the maximum of development, especially the latter as compared to the length of the spine; the thumb, the least constant in the rest of the class, becomes in him the most important, constituting a hand proper; in like manner the great toe, the first obliterated in other mammals, is characteristic of the genus homo, as on it depend principally the erect posturo and biped gait of man; even the highest monkey has a posterior thumb instead of a