CHELONIA. 471 



able to any of the types now represented on the earth. It is 

 still uncertain how the extinct types of Reptiles should 

 be arranged, but many authorities recognise the following 

 orders : — Anomodontia, Sauropterygia, Ichthyopterygia, Di- 

 nosauria, Ornithosauria. To these we shall afterwards refer. 



Order Chelonia. Tortoises and Turtles. 



The body is compact and broad in the region of the 

 trunk. There is a dorsal and a ventral shield, within the 

 shelter of which the head and neck, tail and limbs, can be 

 more or less completely retracted. 



The dorsal shield or carapace is formed from the neural 

 spines of the vertebrae, from the expanded ribs, and from 

 a series of marginal plates around the outer edge. 



The ventral shield or plastron consists of nine plates — a 

 median anterior piece and four pairs of plates behind it. 



Overlapping, but in no way corresponding to the bony 

 plates, are epidermic horny plates of " tortoise-shell," which 

 though very hard are not without sensitiveness, numerous 

 nerves ending upon them. 



The jaws are covered by a horny sheath, and are without 

 teeth, though rudiments of these have been seen in some 

 embryos. 



The average life of Chelonians is sluggish. Perhaps this 

 is in part due to the way in which the ribs are lost in the 

 carapace, for this must tend to make respiration less 

 active. 



All are oviparous. The eggs have firm usually calcareous 

 shells. 



Some Peculiarities in the Skeleton of Chelonia. 



The dorsal vertebra seem to be without transverse processes, and along 

 with the ribs are for the most part immovably fused in the carapace. 

 The tail and the neck are the only flexible regions. 



Dr Berry Haycraft has investigated the development of the carapace, 

 and has been good enough to give me the following brief summary of his 

 results. 



If we compare a very early embryo turtle with that of a crocodile, we 

 notice the following difference : — In the crocodile, each cartilaginous 



