^917-] J- HoRNELL : Indian species of Meretrix. 157 



in their respective markings that unless one has opportunity to 

 examine a large number of individuals from one locality where the 

 various colour forms are present, it can readily be understood how 

 Lamarck and others came to make separate species of simple 

 colour varieties. As long ago as 1835, Deshayes and Milne-Ed- 

 wards pointed out that Lamarck was in error in doing so in regard 

 to what are merely colour varieties of the large species of Meretrix 

 which Linnaeus termed Venus meretrix. They pointed out that in 

 any extensive collection of the large species of Meretrix gradations 

 can be readily traced between Lamarck's species. To quote their 

 own words : — 



" On nous demandera sans doute sur quoi nous nous fondons 

 pour faire de tels changements, et nous repondrons sur 1' observa- 

 tion : en examinant en effet un grand nombre d'individus parmi 

 lesquels se trouvent toutes ces especes de Lamarck, nous avons 

 trouve a la charniere et I'impression palleale des caracteres speci- 

 fiques constants, et de plus nous avons vu de nombreux passages 

 entre les varietes. Dans quelques individus, nous avons meme 

 observe sur une seule coquille les dispositions de couleurs d'apres 

 lesquelles Lamarck avait fait deux especes." 



This conclusion is undoubtedly correct for even in the case 

 of var. impudica, where in the vast majority of cases there is no 

 difficulty in separating at a glance this variety from the type, there 

 are occasional individuals where an intermingling of the colour de- 

 signs proper to impiidica and the type bridges the gap ; others 

 show hybrid markings connecting with varieties morphina and 

 zonaria. 



The least emphatic of the colour varieties is that which I term 

 morphina. Occasionally a shell well-marked with two distinct 

 rays is found, but the variety has none of the stability of impudica 

 and aurora and gradations to the type and to the var. impudica 

 are so frequently met with that it scarcely deserves to be treated 

 separately ; true zonaria is seldom seen in specimens from any 

 Indian locality, but among immature shells of var. impudica, indi- 

 viduals are often to be found with well-zoned bands coinciding with 

 Lamarck's description and the figures by Romer and by Reeve 

 of Cytherea zonaria ; Lamarck's var. gyaphica is also very rare in 

 India and may, I consider, be fused with M. zonaria. 



Habitat 'and distribution. M. meretrix has a restricted and 

 most definite habitat. It is purely an estuarine and backwater 

 species, never found to my knowledge in the open sea. Its home 

 is in the sands adjacent to the main channels leading from back- 

 waters to the sea. It lies buried in a shallow burrow or pit with 

 only the extreme posterior point projecting above the surface 

 when the tide is in, covering these sand flats. At low tide when 

 the sands are uncovered it withdraws entirely below the surface 

 and is then most difficult to locate. 



When young this Meretrix is extremely active, preferring to live 

 in estuarine sands swept by rapid currents. The foot is large and 

 very muscular ; by a variety of movements it is able to make its way 



