REPTILES. 159 



\\ Scales of the belly quadrate, arranged in cross-bands; 

 throat with two cross-folds. . . . TEID^, 73. 



V. FAMILIES OF OPHIDIA. 



* Both jaws fully provided with small teeth; no poison fangs ; 

 no rattle; no anal appendages; no ante-orbital pit; not 

 venomous. ...... COLUBRIIXE, 75. 



** Upper jaw with enlarged, erectile poison fangs, otherwise tooth- 

 less ; a deep pit between eye and nostril ; venomous. 



CROTALID^E, 76. 



*** Upper jaw with small, permanently erect poison fangs; no 

 ante -orbital pit; color red, with black rings; somewhat 

 venomous ....... ELAPIM:, 75. (b.) 



OBDEK T.-TESTUDINATA. 



(The Turtles.) 



Reptiles with the body enclosed between two more or 

 less developed bony shields, which are usually covered 

 by horny epidermal plates, but sometimes (Trionychidce, 

 Spfiargididoe) by a leathery skin. The carapace (upper 

 shield) and plastron (lower shield) are more or less united 

 along the sides. The neck and the tail are the only 

 flexible parts of the spinal column, and these, together 

 with the legs, can usually be retracted within the box 

 made by the two shields. The bony part of the carapace 

 is formed by the dorsal and sacral vertebrae, and the ribs 

 co-ossified with a series of overlying bony plates, usually 

 accompanied by a marginal row. The dorsal vertebrae 

 have their ends flattened and immovably united by car- 

 tilage, and all of them, except the first and last, have 

 their neural spines flattened horizontally so as to form 

 the median line of plates. On either side of this series 

 is a single row of ossified dermal plates overlying the 

 ribs and corresponding in number to the developed ribs 

 of which there are usually eight pairs. 



