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Show 1867.] PROF. HUXLEY ON THE CLASSIFICATION OF BIRDS. 417 pass * are wholly, or in part, bounded by vertebrce provided with thick and expanded ribs ; and these ribs are connected, more or less extensively, on the one hand, with the bodies of these vertebra?, and on the other with the iliac bones. The vertebrae in question, of which there are ordinarily two, constitute the sacrum. In Birds the arches of the vertebrae which correspond with these in their relation to the nerves (and therefore must also be termed "sacral ") give off comparatively slender transverse processes, which seem to answer to those which unite with the tubercles of the ribs in the dorsal region; and it is by these transverse processes only that they are connected with the ilia. 4. The broad and expanded part of the sternum, which immediately follows the coracoidal articular surfaces, receives all the sternal ribs. In all recent Reptilia which possess sternal ribs, some of the latter articulate with narrow prolongations, which extend back from the posterior angle of the expanded rhomboidal sternal plate. The sternum in Birds ossifies in a manner which has not been observed in any Reptile. 6. The ischia never unite in a median ventral symphysis; and both pubes and ischia are directed backwards, approximately parallel with one another and with the spinal column. 7. The proximal constituent of the tarsus is anchylosed with the tibia into one tibio-tarsal bonef ; the distal element of the tarsus similarly unites with the second, third, and fourth metatarsal bones, and gives rise to the tarso-metatarsal bone. The metatarsal of the hallux is shorter than the others, and does not reach the tarsus. Unless, as Gegenbaur has rendered probable, the hind limb of the extinct reptile Compsognathus was similarly modified, these characters are diagnostic of birds. In any case they are highly characteristic of them. 8. Birds have hot blood, a muscular valve in the right ventricle, a single aortic arch, and remarkably modified respiratory organs; but it is, to say the least, highly probable that the Pterosauria, if not the Dinosauria, shared some of these characters with them. The amount of work involved in sustaining a Pterodactyle in the air would seem, physiologically, to necessitate proportional oxidation and evolution of waste products in the form of carbonic acid. If so, a proportional quantity of heat must have been evolved, and there must have been a ready means of eliminating the carbonic acid from the blood. We know of no such means, except those which are afforded by highly developed circulatory and respiratory organs; and therefore it is highly probable that the Pterodactyles had more perfect * The sciatic nerve of the Crocodile is formed, for the most part, by a root which leaves the spinal canal by the intervertebral foramen, interposed between tbe two sacral vertebras, and which passes between the two expanded sacral ribs. It receives a large accessory branch from the preceding, and a smaller from the succeeding, spinal nerve. In Gecko verus the root of the sciatic nerve, which passes out between the two sacral vertebras, is smaller than that which lies in front of it, between the anterior sacral and the last lumbar vertebrae. f See Gegenbaur, I. c. PROC. ZOOL. Soc-1867, No. XXVII. |
