iç)i6.] Fauna of the Chilka Lake : Mollusca Gastropoda, etc. 339 



It is probable that these subfossil species do not all belong to the same period in 

 the history of the lake, though all are undoubtedly recent. The oyster-bed at Ganta 

 Sila (in the presence of Chamo) and the corals on the rocks evidently date from a 

 time when this part of the lake was in direct communication with the Bay of 

 Bengal ; Ganta Sila and the hills near it then forming an island or group of islands 

 in the sea. On the other hand the beds of Area and Meretrix at the head of Rambha 

 Bay mark the position of a channel or creek of later date, probably containing 

 brackish water and representing all that then remained of the sea-passage that once 

 separated the islands from the mainland. The beds may possibly have been laid 

 down when the lake-area, though closed to the south, still remained an open bay with 

 a purely marine fauna ; but doubt is cast on this view by the existence of precisely 

 similar shells in a subfossil condition on the shores of Barkuda I. 



The only case in which we have been able to observe a difference between sub- 

 fossil and living shells is that of Area granosa. The subfossil shells of this species 

 exhibit considerable variety of form (some being much more nearly bilaterally sym- 

 metrical than others) and are never of more than moderate size, the largest having a 

 breadth of 50 mm. The few living examples we obtained were much smaller, the 

 greatest breadth being 26 mm. They differ somewhat in form from any of our subfossil 

 examples in being relatively broader and less inflated (cf. figs. 3-6, pi. xvi). Von 

 Neumayer has described a variety of this species under the name "Area granulosa 

 var. minuta " , ' from a point some distance up the Yang-tse-Kiang river. It was taken 

 with shells of freshwater genera such as Vivipara, Bythinia, Melania and Corbula; the 

 specimens were found in silt and were apparently in a subfossil condition. 1 Our own 

 examples from the main area of the Chilka Lake bear a general resemblance to his 

 figures, but are a little larger and more symmetrical. The variation in A. granosa 

 may thus be compared with that recorded by Bateson in the case of Cardium edule 6 ; 

 but, except in the points noted, we are unable to correlate it definitely with changes 

 in environment. 



SPECIAL CHARACTERISTICS OF THE MOTLUSCAN EAUNA. 



In the general facies of the molluscs of the lake the most noteworthy characters 

 are small size, lack of bright pigment and thinness of shell. 



Among the Gastropods the only shells that commonly attain a length of more 

 than 1 cm. are Nassa labecula, N. marrattii, Pot amides fluviatilis, P. fuscum and Thais 

 carinifera; of these only three occur in the main area, the majority of the shells in 

 this region being less than 5 mm. long. In the case of Eamellibranchs a few fairly 



1 Wiss. Ergebn. Reise Bêla Széchenyi in Ostasien, 1877-80, II, p. 641, pi. i, fig. 4 (1898). 



* In the markets of Shanghai, Soochow and the smaller towns in the same district a dwarfed form 

 of A. granosa is commonly on sale in a living condition. It is said to come from near Ningpo. The 

 shells are covered with fine mud and sometimes bear dead or living Balani. With them I found mixed, 

 in some instances, shells of Cerithiidae and Nassidae of distinctly brackish-water facies. The largest 

 Area shells of this form are about 30 mm. broad and about 20 mm. high. — N. A. ; Soochow : 7-XÜ-15. 



3 Phil. Trans. Roy. Soc, CLXXX (B), p. 297 (1889). 



