340 Memoirs of the Indian Museum. [Vol. V, 



large forms, such as the species of Ostrea, Meretrix, Tapes and Standella, occur in the 

 outer channel and in some cases make their way in small numbers into the northern 

 part of the main area. The only species of even moderate dimensions that occur in 

 the southern part of this area are, however, the almost extinct Area granosa and the 

 species of Modiola and Anatina. 



It is only in a few cases that it is possible to compare individuals from the lake 

 with those from more favourable localities, but in those instances in which this can 

 be done, as in Modiola striatula, Area granosa and Nassa orissaënsis, a distinct 

 dwarfing can be detected. In Area granosa all individuals are affected in the same 

 way, and the dwarfing may be due entirely to changes in salinity, while in Modiola 

 striatula different individuals are influenced in different wa}'s and other causes, 

 such as confined position and periodic desiccation, seem sufficient to account for the 

 results observed (see pp. 362, 363). We have provisionally accepted Mr. Preston's 

 identification of the small Solen common in the lake on a muddy bottom as 5. fonesi, 

 Dunker. If this be correct, the race is evidently dwarfed, for shells of sexually mature 

 individuals are always under 30 mm. in length, whereas specimens of nearly 6 cms. 

 have been found in the sea. Shells from the lake are relatively much broader than 

 any of those noticed by von Martens, who remarks that larger shells are proportionately 

 narrower than smaller ones. Nassa orissaënsis is represented in backwaters near 

 Madras and also in canals of brackish water in the Gangetic delta by a form (var. 

 ennurensis, Preston) with a considerably larger and more deeply sculptured shell ; 

 but it is difficult to see in what respects the conditions in these localities are more 

 favourable. 



None of the shells from the lake, with the exception of those of Modiola, are bril- 

 liantly coloured and dense pigmentation is the exception rather than the rule. Its ab- 

 sence is particularly noteworthy in the Lamellibranchs , among which colourless forms 

 such as the species of Kelly a, dementia, Petricola, Diplodonta, Psammobia, Standella, 

 Theora, Cumingia, Cuspidariä, Lyonsia and Anatina, greatly predominate. Among 

 the Gastropods the commonest colours are dull brown and dull green, as in Vanesia, 

 Litiopa, Nassa, Stenothyra and Potamides. The number of colourless species is com- 

 paratively small, comprising those of Tomatina, Pyrgulina and Chrysallida. The only 

 species in which well-defined and conspicuous markings occur on the shell ' are Neri- 

 tina souverbiana, Tinostoma variegatum and Umbonium vestiarium; even in these the 

 markings are almost microscopic. The only mollusc in which the living tissues are 

 brilliantly coloured is Scintilla chilkaënsis , in which the mantle is yellow and orange. 



In the absence of bright colours the fauna resembles that of fresh water and 

 differs from that of the coast immediately outside the lake, where brilliantly painted 

 species such as Siliqua radiata, Eburna and Sunetta scripta are abundant. The com- 

 plete lack of colour in many of the Iyamellibranchs is doubtless correlated with their 

 burrowing habits. 



It is among the Iyamellibranchs also that thinness of shell is most noteworthy. 



1 Young shells of Meretrix ovum are marked with radiating lines of conspicuous brown spots, but 

 these practically disappear in adults. 



