172 Records 0/ the Indian Museum. [Vol. XIII, 



dimensions have also been worked out to permit of accurate com- 

 parison. 



Millimelres. Respeciive Ratios. 



/' 31X25 =iooto8o'05 



Malacca ... 330X25 =100 to 83 '33 



(M. 10847/2) 1 30x21 =iooto8o-oo 



(^31X25 =1001080-65 



^'"m.^°o8^,7'2) ■'■ 49x39x28 =iooto79-5Qto57-i4 



" S 35x27^x20! =100 to 78-57 to 57-86 



(30X27x21^ =1001090-001072-5 



?4iX3i-X25:^ =100 to 82-93 to 61-58 



I 26^ X 19JX 13 =100 to 73-58 to 49-05 



Tellichen-y , < 37! X 29 X 20^ =100 to 77-33 to 54-67 



j2gX2i|xi4A " =100 to 73-28 to 50-00 



1 33^x26x18^ =100 to 78-20 to 55-64 



/'39-iX3i X2 2^ =100 to 78-48 to 56-96 



..- , 337x32^x22! =100 to 87-16 to 61-49 



Mangalore ... •^29^x241x171 =100 to 82-91 to 60-68 



,271X22^X18 =: 100 to 8257 to 66-06 



Madasara 



( .393X34AX26J =100 to 87-34 to 67-09 



1 38^X34^X25 =1001090-261064-94 



Average of ratios = 100 to 81-49 to 59-69 



The above figures bespeak a very considerable degree of vari- 

 ability among these shells, ranging in the ratios of the three dimen- 

 sions from 100 : 73*28 : 50 to 100 : 90*26 : 64*94 in the case of two 

 extreme forms. The average ratios deduced from the above 18 

 examples work out however at 100 : 8i'49 : 59*69 which are not 

 far removed from those of the type form — 100 : 85*73 : 59'9. 

 This approximation of the averages of the two groups furnishes 

 contributory evidence in favour of the view that their differences 

 are insufficient to warrant separation as distinct species ; at the 

 most, even when we take into consideration the divergences in 

 outline, in the structure of the hinge and in the position of the 

 umbones, the west coast group can be accounted only a variable 

 variety of the type. 



Localities. — The inner sections of tidal estuaries and back- 

 waters on the west coast of India from Cape Comorin to Bombay. 

 Very rarely from the east coast where higher saline conditions 

 favour the predominance of the type form (No. 4792 Ind. Mus. coll., 

 from Tinnevelly, and some among No. 10588/2 from the beach at 

 Vizagapatam ; the latter are however probably of sub fossil origin). 

 It appears again on the western coastline of the Malay Peninsula, 

 typical specimens coming from Malacca and Singapore (Ind. 

 Mus. colln. M. 10847/2 and 10837/2). Further north, a specimen 

 from Arakan (M. 10841/2), size 44X36*75 mm., appears to be 

 intermediate between the type and this variety ; in it the charac- 

 ters of the two seem fairly evenly balanced, but after careful anal}'- 

 sis of each character, I am satisfied it should be assigned to var. 

 ovum. Hence I expect that eventually it will be found that this 

 variety is as characteristic of the whole of the eastern shores of 

 the Bay of Bengal as it is of the west coast of Peninsular India. 



