1?4 THE ANATOMY OF VEETEBEATED ANIMALS. 



7. In all existing reptiles, the ilia are prolonged further 

 behind the acetabulum, than in front of it ; and the inner 

 wall of the acetabuliim is whoUy, or almost completely, 

 ossified. The pubes are directed downwards and for- 

 wards, and, like the ischia, meet in a ventral symphysis. 

 In the extinct Dinosmvria, the pelvis exhibits fonns tran- 

 sitional between the reptilian and the ornithic arrange- 

 ment. 



8. The digits of the pes are not fewer than three ; and 

 the metatarsal bones are not ankylosed together, or with 

 the distal tarsal bones. 



9. In existing rej^tiles not fewer than two aortic arches (a 

 right and a left) persist. Two arterial trunks are given 

 off from the right ventriole, or the part of the single 

 ventricle which answers to it. The venous and firterial 

 cuiTents of the blood are connected, either in the heart 

 itself, or at the origins of the aortic arches. 



10. The blood is cold. There are usually two semilunar 

 valves at the origins of the aortic and pulmonai-y trunks. 



11. The coi-pora bigemina lie upon the upper surface of 

 the brain. 



In Aves, on the contrary: — ■ 



1. The exoskeleton consists of feathers. Ossifications of 

 the dermis are rare, and never take the form of scutes. 



2. In all recent birds, the centra of the cei-vical vertebrae, 

 at least, have subcylindrical articular faces. If, as in some 

 birds, the faces of the centra of the other vertebrae are 

 spheroidal, they are opisthocoelous, which is the rarest 

 ari-angement among reptiles. 



o. The proper sacral vertebrae of birds — that is to say, 

 those between, or through, the arches of which the roots of 

 the sacral plexus pass — have no expanded ribs abutting on 

 the ilia. 



4. The sternum has no costiferous median backward 

 prolongation, all the ribs being attached to its sides. The 

 cartilaginous sternum is replaced, in the adult, by mem- 

 brane bone, and ossifies from two, to five or more, centres. 



