NOTE 



TO 



THE CRAG MOLLUSC A. 



IN the Appendix to the ' Crag Mollusca, 5 p. 323, is the notice of a fossil which I 

 have there assigned, with doubt, to the Genus Aplysia, conceiving it to have been the 

 calcareous portion of an internal shell ; and as it is important that errors of this kind 

 should not remain unconnected, I take the earliest opportunity of making the correc- 

 tion. In the living Aplysia there is a shell or shield situated on the back of the animal, 

 encysted in the mantle, covering the branchial region ; and although this internal 

 shell in the recent state is thin and coriaceous, I thought it possible there might be 

 sufficient calcareous matter in the shell of some species of that genus to permit its 

 being preserved in a fossil state. In this I have been mistaken. Considerable doubt 

 was entertained by me at the time of publication, but it was my desire to have 

 everything figured that appeared to be in any way connected with the Mollusca of the 

 Crag. 



In the course of my examination of the Eocene Bivalves, now preparing for 

 publication, my attention has been directed to the Genus Anomia, and I find there 

 that the right or under valve is sometimes so small as almost to be obsolete or useless 

 as a protection to the living animal, the diameter of the upper valve being in some 

 instances three times that of the lower, and the construction of this latter is often 

 so thin and fragile as to permit the greater part of it to be easily destroyed. From 

 the umbonal region of this valve, proceeding towards the larger side in the interior 

 of the shell, are two thickened ridges, one forming the dorsal margin and the other 

 extending downwards to the body of the shell immediately on the hinder edge of the 

 foramen, which gives a strength and protection to this part of the valve over the 



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