REPTILES. 309 



The vertebrae are mostly procoelous, but some of them may 

 have plain faces. There are two sacral vertebras. 



The position of the girdles inside the ribs is secondary, and 

 is produced during growth by the forward and backward ex- 

 tension of the carapace. The ribs have but a single head, and 

 extend into the caudal region (Fig. 15 5^- The procoracoid is 

 fused to the scapula, the carpus is primitive, but the tarsus is 

 modified by fusion of its ossicles. Five digits always occur, but 

 the number of phalanges is not constant. 



The brain has large hemispheres which cover the twixt and 

 partly the mid brain ; the cerebellum is a slightly arcuate trans- 

 verse fold. The facialis 

 and acusticus nerves are 

 united at their origin. 

 Both Harderian and lach- 

 rymal glands occur, the 

 latter being at the poste- 

 rior angle of the orbit. Fig. 306. ?^Vv\\oi\.m<i\&,Chrysemys picla. 

 T^i . ■ 11 J /', basioccipital; f, frontal; eo, exoccipital; ;', 



The tympanum is well de- . , ■, \ r . 1 



■' ^ jugal; in, maxillary; np, naso-preirontal ; 



veloped, the membrane is /, parietal; pf, postfrontal; /;f, pterygoid; q, 

 visible externally, and the quadrate; s, squamosal; io, supraoccipital. 



Eustachian tube is large. 



The ventricular septum is poorly developed ; the third 

 aortic arches are not connected with the radices aortae, and the 

 left radix gives off the coeliac artery before joining with its 

 fellow. A renal portal system is lacking, the caudal vein 

 connecting with the epigastrics. The sexual and urinary ducts 

 empty into the neck of the urinary bladder. The penis is an 

 unpaired structure arising from the dorsal wall of the cloaca, 

 and in it are two canalicular extensions of the ccelom, which 

 open on two papillas to the exterior. 



The eggs are covered with a leathery calcareous shell, and 

 are buried in the sand, being hatched by the heat of the sun. 



Some of the chelonia are herbivorous, some feed on insects, 

 molluscs, etc., and some are strictly carnivorous. All are rather 

 slow in their motions ; and the group is best represented in the 

 tropics, the colder temperate regions having but few species. 

 In cold climates the species undergo a hibernation, and in the 



