LAND-TERRAPINS. 65 



digits are usually furnished with webs, or at least a rudiment thereof, while the 

 middle toe of each foot has three joints, and the metacarpal bones are elongated. 

 We have first to deal with a small group, mainly confined to the Oriental region, 

 which both in structure and habits tends to connect this section of the family 

 with the preceding one. These forms, as shown in the right-hand figure of the 

 illustration on p. 42, agree with the hinged tortoises in that most of the hexa- 

 gonal neural plates of the carapace have the shorter of the two lateral surfaces 

 placed posteriorly and the longer anteriorly. Moreover, if the horny shields from 

 the plastron be removed, it will be found that the entoplastral, or median unpaired 

 bone of that part of the skeleton, is crossed by the groove marking the boundary 

 between the humeral and pectoral shields. 



spinose Land- The spinose land-terrapin (Geoemyda spinosa) may be taken as 



Terrapin. a well-known example of the first genus, characterised by the absence 

 of a hinge in the plastron, and of a bony temporal arch on the sides of the skull. 

 The three species of this genus are large-sized tortoises, confined to Burma and 

 the Malayan region ; the spinose land-terrapin having a shell of 8 inches in length, 

 while that of the great land-terrapin (6r. grandis), from Burma and Siam, measures 

 upwards of 16 inches. In the former of these two species 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 colour 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. 

 Chaibassa The Chaibassa terrapin (Nicoria tricarinata) figured in the 



Terrapin. illustration on p. 66, and taking its name from a district in Bengal, 

 is 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 5 inches in length, but in a larger 

 one it may measure as much as 16 inches. While in the figured Chaibassa terrapin 

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

 p. 42, 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 colour, 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 w^ell 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. 42, 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. 



VOL. V. 5 



