Characters of Rep' lies. 



FIG. 17. 



more or less perfectly divided by a septum, so as to 

 act as if it were a double chamber (fig. 17). There are 

 always at least two aortic arches, right and left, which 

 usually unite subvertebrally to form one dorsal aorta. 

 The notochord never per- 

 sists in the adult, and in 

 most living reptiles the 

 vertebral bodies unite witn 

 each other by ball and 

 socket joints, and are very 

 rarely biconcave. The 

 skull joins the vertebral 

 column by a single me- 

 dian articular eminence 

 or condyle, and there is 

 no parasphenoid bone, 

 the bones of the middle 

 of the base of the skull 

 being developed in the 

 cartilage of the base itself, 

 not in the membrane be- 

 neath the cartilage. The 

 lower jaw articulates, as 

 in the amphibians, with 

 the end of the preceding visceral arch ; and a bone 

 at its extremity called the quadrate bone is interposed 

 between the palatine part of that arch and the skull. 



Many reptiles are ovoviviparous ; others are ovi- 

 parous. Like the amphibians, the reptiles at the present 

 day, though still numerous, give us a very faint idea of 



Iir former grandeur of size and complexity. In the 

 sozoic age they held the same position on the globe 



Heart of turtle. 

 H, ventricle ; //, k'. auricles. 



