iQiyJ J- HoRNELL : Indian species of Meretrix. 159 



general colour is modified considerably in the majority of indivi- 

 duals by raying, zoning and staining upon the shell substance in 

 the umbonar region, and by a more or less extensive and well- 

 defined dark area on the upper posterior portion of the shell, where 

 it often forms a lanceolate blotch termed the vulva by Lamarck. 

 The colour of this area varies from a dark cloud}- olivaceous 

 brown to a livid purplish brown, exhibiting much variation in tint 

 and in intensity. In all cases in the type the edges of this patch 

 are indefinite, merging insensibly into the adjacent ground colour, 

 ^which is usually rather darker than the remainder of the shell. 



In many individuals the umbones are minutely spotted with 

 pale brown, seen only with the aid of a hand lens. There may 

 also be concurrently a couple of narrow divergent and usually dis- 

 continuous brownish rays. These are sometimes well defined and 

 conspicuous and may be composed of short straight bars or of minute 

 chevrons. This umbonar raying seldom extends further than half 

 an inch from the hinge; beyond that distance the raying either 

 disappears, or in colour variety morphina is continued as two dull 

 and usually diffuse broad bands, having the appearance of a stain 

 within the substance of the shell, rather than surface markings as 

 in the case of the colouring upon the umbones. These distal bands 

 are most variable ; sometimes they form two broad fairly well- 

 defined bands reaching to the ventral margin ; more frequently 

 they are discontinuous and form blotches having only a vague 

 radial arrangement ; often they are entirely wanting. 



In yet another series of colour variations there is distinct con- 

 centric zoning of the umbonar region. This may or may not 

 occur in individuals with spotted and rayed umbones ; there is 

 infinite variety in these combinations. The zones are usually 

 formed by the alternation of very narrow chestnut or livid tinted 

 zones with grey or yellowish ones. 



Romer was of opinion that Lamarck's C. impudica, as repre- 

 sented by the form shown in Romer's figs, i-ic, pi. viii, of a 

 pale coloured shell with a dark vn\v a having sharply defined margin 

 and without decorated umbones, should be regarded as the true 

 colour type of Linnaeus' Venus meretrix. He considered 

 Lamarck's C. meretrix to be a closely connected variation of the 

 type. Taking Romer's pale type and Reeve's C. impudica {loc. 

 cit. fig. 10, pi. iii) with similar vulva and bold chevron-shaped um- 

 bonar markings as representing two colour designs common to the 

 form, Romer claims these to connote the central or true type. I am 

 convinced after examining several hundred specimens that this 

 view is incorrect. By far the greater number, usually slightly 

 over 50 per cent. (c/. next paragraph), are of quite different colour 

 scheme, characterized by a pale indefinite tint with dark vulva 

 'fading gradually at its margin into the ground colour and with the 

 umbones generall}^ minutely dotted with brownish yellow ; the 

 variety coloured as in Reeve's figure of C. impudica is much less 

 numerous, averaging not more than 25 per cent, of the total num- 

 ber. Hence it is more reasonable and convenient to constitute the 



