cxvi Journal of the Asiatic Society of Bengal. [N.S., XII, 



least five strongly characterised varieties approximately co-equal in taxo- 

 nomic value, thus constituting definite co-varieties or sub-species. 



Including Linnaeus' type as one, five varieties were enumerated and 

 defined, namely : — Varieties obtusa, acuta, globosa, comorinensis and 

 fusus. With the exception of the last named, these are new varieties. 



Each of these five forms, if judged by isolated individuals possess- 

 ing the mean of the different characteristics and proportions, may rea- 

 sonably be classed as a distinct species. Study of large numbers — nearly 

 2 millions of these shells are imported into Calcutta annually — shows that 

 this view would be incorrect. 



The following key to the five varieties defines the principal character- 

 istics of each : — k 



/'Shoulder angular, 



-, . , ,.1 prominent var. fusus, Sowerby. 



Spire elongate ; \ r 



shell widely f u si-/ 



form. Breadth in \ / a. Profile of whorls in spire convex, 



length 1.75 to 2 L,^,^ rniirwl var. acuta, var. nov. 



Shoulder round-] h profile of whor]s in gpire nearly 



V ed, low. 



straight, 



var. comorinensis, var. nov 



Z^Spire moderately short ; ^ 



I shell globose ; perios- I vftr hb var . nov . 

 tracum rough and i 



Spire short : shell thick / 



globose or top-l 



shaped. Breadth/ Spire often sh ort;| 



in length, under j shell inclined to be var. obtusa. var. nov., 

 iD ■ top-shaped , very wide V with 2 forms : — 



at shoulder ; periostra- (a) typica 



cum thin in small and I (b) rapa (Gmelin) 



medium-sized shells. / 



The peculiar geographical distribution of varieties obtusa and acuta 

 (including globosa and comorinensis under the latter) is most illuminative 

 upon the influence of differences in environment upon separated groups 

 of an originally united species in stabilizing certain variations from the 

 original stock. 



Reasons were adduced for the belief that originally a single form 

 inhabited the whole of the shallow waters that once extended uninter- 

 ruptedly from what is now Cape Comorin to Madras During this phase, 

 no land connection existed between India and Ceylon. Later, a land 

 barrier was formed along the line Pamban-Rameswaram- Adam's Bridge- 

 Mannar. Two isolated groups of the species were thereby entailed ; these 

 diverged and two very distinct varieties were formed. Had the land 

 barrier not broken down these two varieties would assuredlv have hard- 



| 



ened into separate species. But with the breaking down of the barrier — a 

 very recent geological phase — a mingling of the two varieties, acuta and 

 obtusa, has occurred. In spite of this, the author considers that the two 

 varieties will continue to crystallize their respective characteristics and 

 will end, as they were undoubtedly doing up to the time the land barrier 

 became interrupted, in becoming distinct species. 



The isolation of another section of the stock in the Andaman Island- 

 has similarly resulted in the formation of a variety with well-defined 

 differences from the continental form. 



The paper was an attempt to give a reasoned account of the varieties 

 of one of the dominant molluscs of Indian seas and to offer a working 

 hypothesis for the explanation of the origin of some of the varieties which 



