AN ISLAND IN THE CHILE A LAKE. 269 



Type B. This type is common in a subfossil condition round the Chilka Lake and 

 fresh valves with remains of the periostracum have been found in the Hnnur back- 

 water near Madras. The majority of our specimens from India and Ceylon can be 

 referred to it. The shell is always considerably smaller than in type A^ never exceed- 

 ing 65 mm. in width. It is possible that I have included in it two types really dis- 

 tinct and representing different periods, so far as the Chilka Lake is concerned, in 

 the history of the species, but intermediate individuals occur in the Hnnur backwater 

 and the Chilka deposits have been too much disturbed to afford precise data. In 

 some shells from these beds the outline is subtriangular, with the beak narrow and 

 produced and the breadth slightly greater than the height, but in others the shape 

 is transversely oval, the beak more tumid and shorter and the breadth considerably 

 greater than the height. Typical shells of these two phases are shown in figs. 5 and 

 3 respectively on pi. xvi, Mem. Ind. Mus., V. The periostracum in this form is 

 apparently brownish and the shell sculpture is not strongly developed. Shells of the 

 subtriangular form are, at any rate in some specimens, considerably thicker than 

 transversely oval ones, which are often much thinner than normal. Further informa- 

 tion is desirable about these two forms of shell. 



Type C. The specimens of this type that I have seen are all from the markets of 

 Shanghai and Singapore. At the former place they were said to be brought from 

 Ningpo, and at the latter to be procured locally in the harbour. The shell is much 

 smaller than in type B, more equilateral and broader in proportion to its height. 

 Large specimens are not more than 27 mm. broad and the approximate proportions 

 of those of this size are height to breadth i : i'5, diameter to height i : i'i-i-2, dia- 

 meter to breadth i : 1-35-1 -60. The transverse axis of the shell is also more oblique 

 than in type B and the beak is smaller and less swollen. The sculpture of the ribs 

 is more pronounced and has a nodular character. The periostracum is blackish. 

 The shell is rather thin but not attenuated. 



Type D. My specimens are from the Chilka Lake, the Cochin backwaters on the 

 west coast of India and the market of Moo-too, a village near the Tai Hu Lake inland 

 from Shanghai. Similar shells are sold in the markets of Shanghai and both there 

 and at Moo- too are said to come from Ningpo. They are, however, sold separately 

 from .those of type C. They can be distinguished by their still smaller size, less 

 inequilateral outline, more swollen beaks and almost spiny costular sculpture. It 

 is possible that the Chinese shells (var. mimita Neumeyr) I assign provisionally to 

 this type are merely young of type C, but I believe that those of the Chilka Lake 

 are certainly distinct.' In the latter the characters I have enumerated are more 

 marked than in' the former. The valves are also less swollen and the periostracum 

 pale brown in colour. My specimens, however, are too few to provide reliable data 

 as to proportions. The difference may be seen in a general way by comparing fig. 6, 



' Dr. Ekendranath Ghosh, who is studying the anatomy of the Arcidae, tells me that the sexual organs are 

 mature in a specimen from the Chilka Lake with the shell 9 mm. high, 12 mm. broad and 9 mm. thick. He regards 

 the recent Chilka form as sperifically distinct from the true A. gyanosa. 



