686 PHYLUM CHORDATA : CLASS REPTILIA 



processes, as in Birds ; as in crocodiles, there are numerous " abdominal 

 ribs," ossifications in the subcutaneous fibrous tissue of the abdomen. 

 The anterior end of the " plastron " thus formed overlaps the posterior 

 end of the sternum. The inner ends of the clavicle rest on a median 

 episternum (interclavicle). 



The pineal or parietal eye, which reaches the skin on the top of the 

 head, is less degenerate than in other animals, retaining, for instance, 

 distinct traces of a complex retina (sec Fig. 306). 



Near the living Sphenodon, the Permian Palceohatteria, the Triassic 

 Hyperodapedon, and some other important types may be ranked. 

 Along with these may be included the remarkable Proterosaurus from 

 the Permian, though Seeley establishes for it a special order — Pro- 

 terosauria, as distinguished from Rhynchocephalia. According to 

 Baur, quoted by Nicholson and I-ydekker, "the Rhynchocephalia, 

 together with the Proterosauria, to which they are closely allied, are 

 certainly the most generalised group of all Reptiles, and come nearest, 

 in many respects, to that order of Reptiles from which all others took 

 their origin." 



Second Order : Chelonia. Tortoises and Turtles 



General Characters. — The broad trunk is encased in 

 bones which form a dorsal and a ventral shield, within the 

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

 more or less retracted. The dorsal carapace is usually formed 

 from — (a) the flattened neural spines {plus dermal scutes) ; 

 (b) expanded and more or less coalesced ribs (plus costal 

 dermal scutes) ; (c) a series of dermal marginal scutes around 

 the outer edge. In the Athecce the dorsal vertebrce and ribs 

 are not fused to the dermal plates which form the carapace. 

 The ventral shield or plastron is formed of nine or so dermal 

 bones. There is no sternum. 



Overlapping, but not corresponding to the bony plates, 

 there are {except in Trionychia and Athecce) epidermic 

 horny plates of " tortoise shell,"" which, though very 

 hard, are not without sensitiveness , numerous nerves ending 

 upon them. 



The quadrate is immovably united with the skull. There 

 is only a lower temporal arcade. The jaws are covered by a 

 horny sheath, and are without teeth, though hints of these 

 have been seen in some embryos. There is a single anterior 

 nasal opening. The scapular arch is internal to the ribs. 

 The limbs are pentadactyl, but often in the form of paddles. 



The average life of Chelonians is sluggish. Perhaps this 

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



