CHARACTERS OF REPTILIA. 3 



opistho-coeloiis, or have flat faces ; but their ends are 

 never cylindroidal. 



3. When a sacrum is present its vertebrae have large 

 expanded ribs, with the ends of which the ilia articu- 

 late. 



4. The sternum, when present, is rhomboidal ; and, 

 when many ribs are connected with it, the hindermost 

 are attached to a single or double backward median 

 prolongation. The sternum may be converted into 

 cartilage-bone, but . is never replaced by membrane- 

 bone, and does not ossify from two or more definite 

 centres. 



5. When an interclavicle exists it remains distinct 

 from the clavicles. 



6. The manus contains more than three digits ; and, 

 at fewest, the three radial digits have claws. 



7. The ilia are prolonged farther behind the ace- 

 tabula than in front of them, and the inner wall of the 

 acetabulum is wholly or almost wholly ossified. The 

 pubes are directed ventrally and forward, and, like the 

 ischia, meet in a ventral symphysis. 



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

 the metatarsals are not anchylosed together or with the 

 distal tarsal bones. 



9. Not fewer than two aortic arches persist. Two 

 arterial trunks are given off from the right ventricle, or 

 the part of the single ventricle that answers to it. The 

 venous and arterial currents are connected in the heart, 

 or at the origins of the aortic arches. 



10. The blood varies in temperature with the sur- 

 rounding medium. There are usually two semilunar 

 valves at the origin of the aortic and pulmonary trunks. 



