116 HIGH SCHOOL ZOOLOGY. 



shields on the back and behind the breast bone, which encase 

 the creatures in an ahnost continuous coat of mail. Important 

 differences from the Lizards exist in the endoskeleton ; the 

 great elongation of the skull, e.g., with the formidable teeth 

 lodged in sockets, the form of the nasal cavity, with the anterior 

 nostrils at the tip of the snout and the posterior far back in the 

 mouth, the intimate union of all the cranial and facial bones 

 with each other, etc. Many peculiarities of the other organs 

 point to a gi'eater specialisation than exists in the other Reptiles, 

 thus the brain is of a higher type, and the heart is subdivided 

 into four compartments, although there is still a certain mixture 

 of the venous and arterial blood immediately outside it. The 

 order is oviparous like most Reptiles, the eggs, which are very 

 fragile from the small percentage of lime in the shell, being laid 

 in the sandy banks of the streams in which they live. 



22. Although the Crocodilia hardly number more than twenty 

 species at the i)resent day, yet the Fossil species are far more nu- 

 merous than is the case in any other of the four orders of livirg 



Fiff. 84 — Restoration of Plesiosaurus. ^. 

 Reptiles. The earliest of them had amphicoclous vertebrre, and 

 a much more complete exoskeleton than the living Crocodiles, 

 probably for protection against the gigantic aquatic Reptiles 



