68 
Memoirs of the Indian Museum. 
[Voi,. II, 
among some other families of Cirripedes (for instance, in the genus Balanus in the 
Balanidæ) and I cannot find a very good example among the Indian species of the 
I,epadidæ, 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 geryonopJnla. 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 eas}^ 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 Laccadive Sea 
to the south and the Persian Guh to the north may be conveniently regarded. Until 
recently the two subfamilies Alepadinæ and Oxynaspidinæ 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 Alepadinæ 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 Lepadidæ 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. 
