1916.] Fauna of the Chilka Lake ; Mollusca Gastropoda, etc. 361 



that in a small series of M. lacustris, von Martens, from the Tung-ting Lake in China, 

 we find variations in the shape of the shell comparable to those that occur in the spe- 

 cies from the Chilka Lake. 



M. striatula differs from M. undulata so far as habits are concerned in that it is 

 usually found attached to rocks, stones, wooden posts or other solid objects. This 

 is the case in the Gangetic delta as well as in the Chilka Lake. Shells are occasion- 

 ally found in both places fastened to algae growing on stone, but seem to be unable 

 to attain their full development in this position. In the Gangetic delta a favourite 

 situation is on posts partly destroyed by Xylotrya ; but the mussel is also found on 

 brick-work in the Calcutta docks, where it is stated to do considerable damage by 

 settling in cracks in the bricks and splitting them by its growth. In the Chilka Lake 

 it prefers to settle in crevices in rocks or among oyster-shells. We have noticed 

 on many occasions that the young molluscs show a marked tendency to congregate 

 round the adults (pi. xvi, fig. 2). 



In the lake it is extremely abundant in both the outer channel and the main 

 area and occurs at all times of the year, being very common in all suitable places 

 whenever the rocks and oyster-beds are covered with water. 



Near Calcutta, where it is very abundant, it is frequently overwhelmed by the 

 sponge, Spongilla alba, in which we occasionally found shells in the Chilka Lake. On 

 the bottom of our steam-launch large numbers were also discovered in the sponge 

 Sub er it es sericeus. 



The shells that we have included under the name M. striatula vary very greatly 

 in shape, sculpturing, size and colouration, but we find from the old collection of 

 the Indian Museum that all, or practically all, were included by G. Nevill under the 

 name M. emarginata, Benson. This name seems to have existed in manuscript some 

 time before it was published by Reeve, and it was the one by which the common 

 mussel of the estuaries of the Bay of Bengal was known to Blanford l and his con- 

 temporaries. Nevill gives striatula as a synonym on his labels. 



From M. undulata the species appears to be distinguished by the following charac- 

 ters, though in certain cases we have found it very difficult to separate individual 

 shells of small size. The shell is always more opaque and as a rule much more 

 densely pigmented, the pigment being of a duller shade. The upper margin is as a 

 rule less strongly elevated and more evenly arched, the proportional depth of the 

 shell being therefore less. The postero-dorsal margin is as a rule more declivous and 

 the posterior extremity more narrowed and less strictly horizontal. In a large number 

 of shells the ventral margin is boldly excavated or emarginate. Radial ridges, which 

 are exceptional in M. undulata, are usually present ; but the anterior margin is some- 

 times quite smooth. 



The nominal species that we include under M . striatula may be divided into two 

 groups, (i) those in which the lower margin of the shell is practically straight and (ii) 

 those in which it is distinctly excavated. The former consists of M. cochinensis and 



1 Blanford, however, distinguished some specimens as M. striatula. 



