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



that of the species of Diplodonta, living molluscs were found in the salt-water season, 

 but only dead shells in that of fresh water. 



Several species, notably Cardium (Fulvia) rugatum and Mytilus smaragdinus , 

 evidently make their way at a young stage from the sea into the outer channel, but 

 are unable to survive until maturity ; they must be classed merely as occasional 

 visitors of no faunistic interest in so far as the lake-fauna is concerned. 



Many species belonging to freshwater genera, such as Ampullar ia , Vivipara and 

 Planorbis, are very abundant in rice-fields and even in small pools of rain-water near 

 the margin of the lake ; but we did not observe a single instance in which molluscs 

 of this kind made their way into the lake itself, even when its waters were quite 

 fresh. This fact is particularly remarkable in the case of Melania tuberculata, which 

 is common in pools of both fresh and brackish water near Rambha and occurs in 

 great abundance in water of considerable salinity in the Gangetic delta. 



SUBFOSSIL SHELLS. 



The late Dr. W. T. Blanford drew attention in 1859 to the fact that there were 

 large beds of subfossil estuarine molluscs in the neighbourhood of Rambha. The 

 species best represented in these beds, as he noted, are Area granosa, Linn., and Mere- 

 trix casta, Chemn. Thais carinifera is also fairly common. Worn shells of A. granosa 

 and M. casta are also very abundant on the shore of Barkuda I.; the latter species, 

 though common in the outer channel, is now extinct in the main area, in which 

 A. granosa is very scarce. 



Another species found in the main area in a subfossil condition is the common 

 ' f window-pane oyster " , Placuna placenta (Linn.), beds of which, of very limited extent, 

 were proved to have existed near Samal I. and at other points. This mollusc no 

 longer lives in any part of the lake, though it is collected for commercial purposes in 

 lake Tamblegam (Tampalakaman), a smaller lagoon on the coast of Ceylon, the 

 water of which probably also undergoes great seasonal changes in salinity. 1 A 

 detailed comparison of the conditions in the two lagoons in this and in other respects 

 would be of great interest. 



We have already alluded to the young oysters occasionally found on rocks in the 

 main area; at the southern end of the lake single valves, evidently long dead, were 

 frequently observed, while at the edge of the water near Ganta Sila we found the 

 remains of an oyster-bed. The species {0. virginiana) was the same as that now 

 found living at Manikpatna, but the beds differed in that shells of the genus Chama 

 were abundant on the oysters. On the rocks at the same place skeletons of solitary 

 corals belonging to the family Turbinolidae were occasionally seen (pi. xiv, fig. 3). 

 Chama was not found on the Manikpatna beds, but is usually associated with oysters 

 taken in shallow water off the coast of Orissa, while the Turbinolidae are characteristi- 

 cally marine and are particularly abundant off the same coast. 



1 Hornell, Ceylon Marine Biol. Reps., I, p. 41 (1906). According to Mr. Hornell the specific gravity 

 of the water of this lake in the dry season varies from 1-015 to ro.19 at temperatures from 86° to 90 F. 

 No observations have been made as to the conditions in the wet season. 



