476 Mr. W. Clark on the Classification of the 



gans^ and having removed the Brachiopoda, which custom has 

 placed at the head of the bivalves, to a position of less pretension, 

 it has become necessary to invert nearly the usual order of ar- 

 rangement, that animals of similar relations may be associated. 

 This change entirely hinges on, and is the result of, the transfer- 

 ence of the Brachiopoda from the position they have so long oc- 

 cupied ; otherwise the ancient distribution would have been nearly 

 as satisfactory. But the false position of the Brachiopoda, ac- 

 cording to our views, has admitted of no alternative. 



In conformity with these observations, the Anomim, Ostrece, 

 and Pectines naturally follow the Brachiopoda with which they 

 have relations, and are succeeded by the Mytilidse, &c., and 

 brought, according to the intervening genera of the synopsis, to 

 the Gastrochsenidse ; the remaining families of the Myadse, Sole- 

 nidse, and Pholadidse, are thus placed at the head of the list and 

 fonn a very natural group ; and I think that their decidedly 

 higher organization — I particularly allude to the Pholades — and 

 superior functions, as those of excavation, together with the com- 

 pound structure of their shells, as is evidenced by the complica- 

 tion of the accessorial appendages as well as the consideration of 

 the increased importance of the siphonal tubes and the envelop- 

 ing mantle, bring them by these advances in composition into 

 closer connection with the Gasteropoda than with the Ascidise, in 

 the vicinity of which they have been placed from their muscular 

 siphonal sheaths and closed mantle, which have been considered 

 to bear a resemblance to the coriaceous envelopes of those ani- 

 mals. We have no difficulty in admitting Venerirupis into the 

 family of the Gastrochsenidse, though, by the teeth, it is allied to 

 the Veneres, but we consider the character of the teeth of very 

 inferior value to the closed mantle, which points out its natural po- 

 sition so clearly as to admit of no discussion. Teredo terminates 

 the Acephala and passes them to the Dentaliadse, our primary 

 family of the Gasteropoda, agreeably to the indicia that are 

 pointed out in the last page of the anatomy of Teredo. 



Third Division. 



r lateribranchiata . 

 Gasteropoda < cyclobranchiata. 

 {^cervicobranchiata. 



The animals of this division are strict hermaphrodites without 

 congression. The Dentaliadse ai'e the Lateribranchiata of the 

 synopsis, of which family I have already given in the 'Annals of 

 Natural History ' a detailed anatomy ; they have claims which 

 appear not to be ill-founded, to stand as the first family of the 

 Gasteropoda, from the connection between them, by the position 



