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



Tinostoma variegatum* Preston , 19 14, p. 302, figs. 6, 6a, 6b. 

 A few specimens (including the type) were obtained at Manikpatna in the outer 

 channel in March. 



Family Trochidae. 



Umbonium vestiarum (Iyinn.), Preston, 191 5, p. 297. 



Several specimens were found in the outer channel in the salt-water season on 

 sandy ground near the mouth of the lake. 



Solariella satparaënsis ,* Preston, 1914, p. 302, figs. 11, 11a, lib; 1915, p. 297. 

 This species occurs in the outer channel in the vicinity of Satpara and Barham- 

 pur I. at all times of the year, but is rather scarce. 



Class LAMELLIBRANCHIATA. 



Order TETRABRANCHIATA. 

 Family Ostreidae. 



Ostrea virginiana, Gmelin, Reeve, Conch. Icon., XVIII, Ostrea, pi. vi, fig. 9 ; 1873 

 (as 0. rostrata). Plate xiv, fig. 2. 



We have already discussed the beds formed by this oyster in the outer channel 

 of the lake (p. 336). It is an extraordinarily hardy species and can endure desicca- 

 tion, even when exposed to a tropical sun, for considerable periods as well as immer- 

 sion in fresh water. We have also observed it living in similar conditions in back- 

 waters near Madras, at the head of Port Blair harbour in the Andamans and in a 

 lagoon on the Gulf of Siam. 



We have to thank Mr. E. Vredenburg of the Geological Survey of India, who is 

 engaged in a study of the oysters of this group, for the information embodied in the 

 following note. 



The specimens from the four localities referred to above seem to belong to a 

 smaller race of a very large oyster (probably the largest living species) which is known 

 to occur abundantly at many points along the south coasts of Asia , from the Mekran 

 to the Malay Peninsula. Mr. Vredenburg is of opinion that, taking into account the 

 variability in the shape of oysters generally, it is not possible to discover in the shape, 

 the build, the ornamentation, the proportions or the dimensions of these shells, any 

 differences sufficiently precise to afford an excuse for separating the form specifically 

 from Ostrea virginiana, Gmelin, a very common shell along the Atlantic coast of 

 North America. The species seems to be practically cosmopolitan throughout the 

 warmer seas. 



Certain forms occurring in the Bay of Bengal, including that named 0. gvyphoides 

 var. cuttackensis and found on the Orissa coast, were separated from Ostrea virgi- 

 niana by Messrs. Newton and Smith (Rec. Geol. Surv. Ind., XIJI, pp. 1-15 ; 1912) 

 (a) on account of differences in shape or build which Mr. Vredenburg considers 

 inadequate for specific distinction in such variable organisms, and (b) on account 

 of the absence of the deep purple-black or purple-brown colour which, in North 



