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Show 416 PROF. HUXLEY ON THE CLASSIFICATION OF BIRDS. [Apr. 11, 6. The mandible is connected with the skull by the intermediation of a quadrate bone (which represents the incus of Mammalia). 7. Each ramus of the mandible is composed of a number of separate ossifications, which may amount to as many as six in all. (Of these the articulare represents the malleus of Mammalia). 8. The apparent " ankle-joint" is situated not between the tibia and the astragalus as in the Mammalia, but between the proximal and the distal divisions of the tarsus*. 9. The brain is devoid of any corpus callosum. 10. The heart is usually provided with two aortic arches; if only one remains, it is the right. 11. The red blood-corpuscles are oval and nucleated. 12. The cavities of the thorax and abdomen are never separated by a complete diaphragm. 13. The allantois, which is highly vascular, is very large, and envelopes the embryo; but no villi for placental connexion with the parent are developed upon it. 14. There are no mammary glands. I attach less weight to the first of these characters than to the rest, since the simpler kinds of feathers very closely approach hair in structure and development; but the other thirteen are, for the most part, of extreme importance, and define Birds and Reptiles, as a whole, very sharply from Mammals. Closely as Birds approach Reptiles, however, and small as the divergence of the ornithic type from the reptilian appears to be, in view of the great divergences of Reptiles from one another, there are still a number of characters common to Birds which are absent in all recent Reptilia, and, so far as our knowledge goes, in extinct Reptiles-though it must be carefully borne in mind that our information respecting the latter is limited to an acquaintance with their osteology. Thus- 1. Birds possess epidermal appendages developed in sacs of the dermis, and having the structure of feathers. 2. More or fewer of the anterior vertebrae have centra with cylin-droidal articular surfaces f. 3. Although all birds possess a remarkably large sacrum, the vertebrae, through the intervertebral foramina of which the roots of the sacral plexus (and, consequently, of the great sciatic nerve) pass, are not provided with expanded ribs abutting against the ilium externally, and against the bodies of these vertebrae by their inner ends. In recent Reptiles, possessing well-developed hind limbs, the intervertebral foramina through which the roots of the sciatic nerve * See Gegenbaur,'Archiv fiir Anatomie' (1863), and ' Untersuchungen zur yergleichenden Anatomie' (1864). t Archceopteryx may possibly prove an exception to this rule. When certain of the vertebras of Birds (as in the Penguins, Larus fuscus, and others) have centra with spheroidal articular surfaces, the anterior faces of the centra are convex and the posterior concave, which is the rarest case among the Reptilia. The proccelous form of vertebra, so common among the Reptilia, has not been observed in the cervical or dorsal regions of the spine of Birds. |