14 EMYSTERRAPIN. 



vertebral plates has a protuberance; that of the fourth is the most prominent; 

 these tubercles, taken together, give an obtusely carinated appearance to the shell 

 — some varieties have the prominences but slightly developed. Of the lateral 

 plates, the anterior is irregularly quadrilateral, largest at its lovs^er and anterior 

 border, where it is joined to four marginal plates; the second and third are pen- 

 tagonal; and the fourth very irregularly quadrilateral. The marginal plates are 

 twenty-five in number; the nuchal, or intermediate, varies in shape — sometimes 

 triangular, with its apex truncate and directed anteriorly; at others it is nearly 

 quadrilateral; the remaining marginal plates are nearly quadrilateral; the five 

 posterior are slightly revolute, and thus form at times a sort of gutter. 



The sternum is sub-oval, entire in front, and extending even as far as the shell; 

 it is emarginate behind, and does not reach the length of the carapace, which is 

 covered with twelve plates marked with concentric striae. The gular plates are 

 triangular, with their apices directed backward; the brachial are nearly quadri- 

 lateral, the lateral margins most extensive; the thoracic and abdominal plates are 

 oblong squares, and the sub-caudal lozenge-shaped. Of the supplemental plates, 

 the axillary are irregularly quadrilateral and broadest before, while the inguinal 

 are triangular and broadest behind. 



The head is very large, and broadest posteriorly; narrow, and almost pointed 

 in front; above, it presents a smooth surface, as if the head were covered with a 

 single large rhomboidal plate, differing in some degree in colour from the rest of 

 the animal. The jaws are strong and cutting, the superior slightly emarginate; 

 the inferior curved in front, furnished with a hook. The eyes are small, the pupil 

 black, the iris gray, approaching the colour of the skin. The neck is short and 

 thick. 



The anterior extremities are moderately long, and scaly before; scaly and granu- 

 late behind; the fingers are five, webbed, and each furnished with a short strong 

 nail. The posterior extremities are rounded above, but flattened at the tarsus; 

 scaly in front and granulate behind; there are five toes fully webbed, but four only 



