2422 TORTOISES, TURTLES, AND PLESIOSAURS 



both the front and hinder margins of the shell are deeply serrated; whereas in the 

 latter, as well as in the third representative of the genus, only the hinder border is 

 thus ornamented. The color of the carapace in these terrapins is brown or blackish, 

 frequently with darker markings. Together with the other members of the group, 

 they differ from the majority of the terrapins in having the head covered with a 

 continuous skin, instead of with small shields. The small size of the webs of these 

 terrapins 'indicates that in habits they are probably in part aquatic and in part 

 terrestrial. 



The Chaibassa terrapin (Nicoria tricarinata) figured in the illus- 

 tration below, and taking its name from a district in Bengal, is 

 Terrapin 



selected to represent a genus common to the Oriental region in the 



east, and Central and South America in the west, distinguished from the preceding 

 by the presence of a bony temporal arch to the skull. Of the seven species of this 

 genus, the smallest (here figured) has a shell of only five inches in length, but in a 

 larger one it may measure as much as sixteen inches. While in the figured Chai- 



CHAIBASSA TERRAPIN. 

 (Two-thirds natural size.) 



bassa terrapin both fore and hinder margins of the shell, as shown on the left-hand 

 figure on p. 2399, are entire, in other species either one or both of these may be 

 deeply serrated. The Chaibassa species, which ranges from Bengal to Assam, has 

 the carapace dark brown or black in color, with the three longitudinal ridges 

 from which it takes its name yellow; the plastron being uniformly yellow and the 

 neck and limbs blackish. From the larger three-keeled terrapin (N. trijuga) of 

 India and Burma, this species is further distinguished by its more convex shell, 

 which descends very abruptly behind, as well as by the rudimentary condition of 

 the webs between the toes; on both of which grounds it may be regarded as more 

 exclusively terrestrial in its habits. A fossil shell of the Chaibassa terrapin, 

 represented in the right figure on p. 2399, has been obtained from the Pliocene 

 rocks of the Siwalik hills of Northern India, thus indicating the extreme antiquity 

 of the species. In some individuals the hinder half of the plastron is connected 

 with the upper shell merely by ligament. 



The third genus of this group .( Cyclemys) , which is confined to India, Ma- 

 layana, and the south of China, is represented by some half-dozen species, which, 



