1915.] Fauna of the Chilka Lake : Crustacea Decapoda. 231 



Family GRAPSIDAE. 



In addition to purely marine species this family includes numerous forms char- 

 acteristic of backwaters and estuaries, some aquatic, some amphibious and some 

 almost wholly terrestrial. A considerable number of species are known to exist in 

 water of low salinity and some have succeeded in establishing themselves in fresh 

 water. In the Andaman Is. for instance, Ptychognathus andamanicus, Alcock, lives 

 in streams far above tidal influence, while Sesarma thelxinoë, de Man, was described 

 from an altitude of 700 ft. In the case of the Andamans, migration from salt to 

 fresh water is perhaps more easily accomplished than in other parts of India, for 

 Potamonidae are entirely absent and the Grapsids are not therefore brought into 

 direct competition with other crabs. As is pointed out on p. 233 the presence of 

 Potamonidae appears to play an important part in hindering Varuna litterata from 

 establishing itself in Lower Bengal. 



It is remarkable that the genus Metaplax, which is abundant in the Gangetic 

 delta and also occurs in backwaters near Madras, is not represented in the fauna of 

 the Chilka Lake. 



Subfamily GRAPSINAE. 



Genus PACHYGRAPSUS, Randall. 



Pachygrapsus propinquus, de Man. 



1908. Pachygrapsus propinquus, de Man, Rec. Ind. Mus., II, p. 216, pi. xviii, fig. 2. 



Although many of the specimens of this species obtained in the Chilka Lake are 

 very much larger than the types, they do not differ in any marked features from them 

 or from the exhaustive description which de Man has supplied. 



The anterior part of the gastric region of the carapace and the frontal lobes are 

 beset with small tubercles which, posteriorly, tend to form transverse ridges much 

 more conspicuous than in the types. Adults resemble the original specimens in 

 having the inner margin of the ischium of the outer maxillipedes quite straight. 



The chelipedes in the largest individuals are a little unequal- they are, however, 

 identical in structure. The inner edge of the merus bears three or four blunt tuber- 

 cles at the base and projects distally as a thin crest bearing three or four teeth that 

 decrease in size as they approach the carpal articulation. The spines at the distal 

 end of the lower margin of the merus of the ambulatory legs vary in number from 

 two to four; the dactyli in all are conspicuously shorter than the propodi. The pro- 

 podus in the penultimate pair is three and a third times as long as broad, the dac- 

 tylus being about two-thirds its length. 



The largest individual is a female in which the carapace is 23*0 mm. in length 

 and 28-8 mm. in breadth. In the largest male the length is 16-4 mm. and the 

 breadth 20 - o mm. 



The colour of the species when alive was striking. The dorsal surface of the 

 carapace was dull olive, boldy mottled with dark purple. The chelipedes were deep 

 violet, shading to orange red on the fingers, while the ambulatory legs were olive 



