168 THE ANATOMY OF VERTEBRATED ANIMALS. 



ischia, meet in a ventral symphysis. In the extinct Dino 

 sauria, the pelvis exhibits forms transitional between the rep- 

 tilian and the ornithic arrangement. 



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

 metatarsal bones are not anchylosed together, or with the distal 

 tarsal bones. 



9. In existing reptiles not fewer than two aortic arches (a 

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

 from the right ventricle, or the part of the single ventricle 

 which answers to it. The venous and arterial currents 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 pulmonary trunks. 



11. The corpora higemina 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 cervical vertebrae, 

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

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

 roidal, they are opisthocoelous, which is the rarest arrange- 

 ment among reptiles. 



3. 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 pro- 

 longation, all the ribs being attached to its sides. The carti- 

 laginous sternum is replaced, in the adult, by membrane-bone, 

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



5. When an intercla\dcle exists it is confluent with the 

 clavicles. 



6. The manus does not contain more than three digits, and 

 not more than the two radial digits have claws. 



7. The ilia are greatly prolonged in front of the acetab- 

 ulum, the inner wall of which is membranous. The pubes 

 and ischia are directed backward, more or less parallel with 

 one another, and the ischia never meet in a ventral symphysis. 



8. The astragalus sends up a process on to the'front face of 

 the tibia, and early anchyloses with the latter bone. In this 

 character, Birds differ from all existing Reptiles. The foot 



