Dr. J. E. Gray on the genus Assiminia. 183 



The articulations which separate the calcareous pieces of the 

 sclerobase of Isis, consist of corneine. The sclerites of Isis are 

 0*18 to 0"20 mill, in length, with a swelling at each extremity; 

 they are of a fine white colour. 



The substance of the axis of Pennatula phosphorea has been 

 analysed by M. Fremy. It contains a considerable quantity 

 of phosphate of lime with the carbonate of lime. These are 

 the only Radiated animals containing phosphate of lime, and 

 this pecularity was the less expected as the axis of Virgulana 

 and Pavonaria contain only carbonate of lime. 



XVII. — On the Genus Assiminia. 

 By Dr. J. E. Gray, F.R.S., Y.P.Z.S. 



To the Editors of the Annals of Natural History. 



Gentlemen, 



In the preceding Xumber of the ' Annals ' Mr. Clark thinks he 

 has proved that Dr. Leaches genus Assiminia is only a species 

 of Truncatella of Risso. Mr. Clark^s description proves the con- 

 verse of his position. Truncatella should have a subcvlindrical 

 shell with a slender tapering tip, which falls off when the shell 

 approaches adult age ; hence the name of the genus : Assiminia 

 has a broad conic shell with an acute tip which does not fall off ; 

 if it is to be a species of the same genus, the name of the latter 

 ought to be changed. 



The foot of Truncatella is small and peculiarly formed, and the 

 eyes of all the species, according to Mr. Clark's observations, are 

 large with a white iris ; now this is not the case with Assiminia 

 and yet Mr. Clark regards it as a Truncatella. 



Every naturalist has the right to restrict his genera as he 

 pleases. I have only to observe that Mr. Clark's notions on this 

 head are not those usually held by modern zoologists, and this 

 must explain the proposed union ; it is not so extraordinary as 

 that of the species which he has combined together under the 

 generic name of Murex in his late work, species which are by 

 other authors referred to six distinct families. If we were to ex- 

 tend the views of Mr. Clark as applied to the British Mollusca 

 to the exotic species, that is to say to the ilollusca known, many 

 of our genera would contain from 500 to 700 species, which cer- 

 tainly would not facilitate the study of these animals. But the 

 whole of Mr. Clark's theory is so opposed to sound logic, that I 

 shall not proceed further. 



I am, Gentlemen, yours truly, 



J. E. Gray. 



