68 Memoirs of the Indian Museum. [Voiv. II, 



among some other families of Cirripedes (for instance, in the genus Balanus in the 

 Balanidae) and I cannot find a very good example among the Indian species of the 

 I.epadidse, for in cases in which variation is most common and most extreme it is 

 very difficult (and therefore inadvisable) to subdivide species at all, one of the first 

 requisites of a sound classification being that it should be free from unnecessary 

 complexities. The best example of a variety among the species described in this 

 paper is the var. fissicarina of Dichelaspis geryonophila. This form has the carina 

 split transversely, while the valve is entire in the typical form of the species, which 

 occurs side by side with fissicarina on the same crab. 



Distinct as the two kinds of groups into which species may be subdivided really 

 are as regards origin, it is not always easy to say whether a group should be regarded 

 as representing a variety or a race. The only way to settle, such a question is to 

 examine a very large number of specimens from different localities and districts that 

 have been recorded accurately. Few museums possess collections of sufficient scope 

 to render this possible, and even in cases in which it has been possible to examine 

 series including hundreds of specimens from one or several localities, such series rarely 

 represent more than one region or district. A case in which a difficulty of the kind 

 arises is that of the forms assumed by Conchoderma virgatum. To some extent these 

 several forms appear to have a geographical significance, but individuals intermediate 

 between them occur in the same localities as other individuals that agree exactly with 

 local types, and in any case the number of specimens examined by any one specialist 

 has been comparatively small. There are doubtless European and American museums 

 in whose collections fairly large European and American series of the species are 

 included, while the Indian Museum contains a very fair series of Indo-Malayan speci- 

 mens ; but it has not been possible to make arrangements to compare all the specimens 

 already preserved in collections, much less to collect sufficient material from widely 

 separated stations for a proper study of the species as a whole. 



In preparing this paper, therefore, I have thought it best to restrict my attention 

 so far as possible to species occurring in the Bay of Bengal (including the Andaman Sea 

 and the Gulf of Manaar) and the Arabian Sea, as parts of which the I^accadive Sea 

 to the south and the Persian Gulf to the north may be conveniently regarded. Until 

 recently the two subfamilies Alepadinœ and Oxynaspidinae were not known from within 

 these limits, but during the last few weeks a species of Oxynaspis has been found 

 at the head of the Bay of Bengal in considerable numbers, and there can be little 

 doubt that representatives of the Alepadinse will also be discovered at some future 

 date. In the meanwhile, however, I cannot, through want of material, deal with this 

 subfamily. 



The greater number of the Indian species of I^epadidse have already been well 

 figured, either in the Illustrations of the Zoology of the R.I. M.S. ''Investigator'' or 

 elsewhere. I have therefore been forced to economize as regards illustrations and 

 to do without detailed figures. I am much indebted, however, to the draftsmen 

 of the Indian Museum and the Indian Marine Survey for assistance in preparing the 

 outline figures here reproduced. 



