OP SOUTHERN INDIA. S 



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MoUusca,* but it also attains this in a way wliicli makes due reference to the 

 modification of one of the most important- the locomotive- organ of the animals. 

 And this is what led me to decide for the name pelectpoda in preference to any 

 other, tliough I am well aware that it has not yet come into general use, but, 

 I believe, it has been unduly neglected. I have given my argument in favor 

 of it, and leave the rest to the opinion of more able classificators than myself. 



The class Pelecypoda has been variously sub-divided by authors on Conchology 

 or Malacology. The jn'opositions of the different systems are recorded in almost 

 every malacological treatise, and there is not, therefore, any sufficient reason that I 

 should note them here at any lengtli. The usual classification is that in di- and 

 MONO-MYARiA, the former being sub-divided again into sinu- and integropalliata. 

 Deshayes in his last edition of the Paris fossils accepts the general term 

 MOLLuscA ACEP3ALA, and dcfincs three sub-classes — acephala bimyaria, acepkala 

 MONOMYARiA, and BRAcmopoDA. The last is very different in organization, and 

 should be left out altogether from the present field of observation. 



I have carefully considered Deshayes' observations regarding tbe differences 

 wbicb he points out in the muscular and nervous systems of his monomyaria, as 

 compared with those of the dimyaria, and, I believe, tbey will not be greatly sup- 

 ported by the thorough anatomical examination of the animals ; they certainly do 

 not apply to several of the monomyaria wbicli I have examined. The distinction 

 as to the presence of one or two retractile muscles appears to me to be not an 

 essential one ; the 3IYTILA CEA clearly show that the distinction cannot be 

 maintained, with the result of a rational classification. Neither do I find the differ- 

 ences pointed out in the nervous system between the two divisions confirmed. Quite 

 the same remark applies to the supposed distinction of sinu- and inteoro-palltata. 

 There is barely a single family of Pelecypoda in which we would not find both the 

 groups represented ; take, for instance, Ilya, Tugonla, and Cnjptomya ; Lutraria 

 and Anatinella ; Bonax and Tancredia ; Cytherea and Dosinia ; Trapezium and 

 CoralUophaga ; Nuculana and Nncula ; the various forms of Dreissena and 

 3IytUus, &c., &c. I do not see the benefit of these divisions ; and I believe that 

 the most natural course we can adopt is to groiip the Pelecypoda according to the 

 principal types, which represent, so to say, the starting points, or centres, of the 

 variously developed or organised forms. This mode of classification, being based 

 at the same time upon tlie genealogical development, has been found to be the 

 most successful also in the other large divisions of the animal kingdom, though I 

 readily admit that the great difiiculty in making this classification really a natural 

 one rests in the discovery and correct definition of those so-called principal types. 

 In the present state of our knowledge of the Mollusca, we cannot pretend to say 

 that we are already in possession of the materials for such a successful natural 

 classification, and every attempt towards it must, therefore, be looked upon only 

 as such. 



* Ccijlialoijoda, Giistropoda, Pckrypoda, Crarliiopoda, Saccopoda, Ciliipoda,— see introduction to my Monoyi-apli of 

 tbe South Indian Cictaccous Gastropoda, Pal. Ind., Vul. II. 



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