FOSSIL.SHELLS OF THE CRAG. 231 



Coralline crag, Ramsholt and Sutton. 



I propose to give the generic name Pleurodon, to a small 

 coralline-crag shell, which it seems to me cannot properly be 

 referred to any genus hitherto established. The Nucula 

 miliaris of M. Deshayes, if not specifically identical with this 

 shell, must be closely allied to it ; and in placing the French 

 fossil in the genus Nucula, M. Deshayes has expressed a 

 doubt as to the correctness of his determination, in conse- 

 quence of the existence of the lateral tooth. The affinities 

 of Pleurodon, as indicated by the shell alone, probably justify 

 its being placed with the Arcacece ; though its precise posi- 

 tion in that family cannot be determined in the absence of a 

 knowledge of the animal. 



At Ramsholt the valves are generally found united, the 

 large prominent teeth retaining them in apposition. I have 

 given as a synonyme the above name of Deshayes, presum- 

 ing his Nucula miliaris may be the same shell, but I am un- 

 able to discover the internal pit for the ligament he speaks of, 

 and which his figure so conspicuously displays. The crag 

 specimens have a depression placed externally to the teeth 

 on the shorter side, where I imagine the ligament to have 

 been situated. Deshaye's figure shows seven teeth, whilst 

 the shells from the crag never display more than six, and sel- 

 dom more than five. 



Area No(B, Auct. PI. xiii. fig. 2 and 2 a. 



Red crag Sutton, fig. 2. Coralline crag, Sutton, fig. 2 a. 



T feel unwilling to give a new name to this shell, since a 

 very scrupulous examination inclines me to the behef of its 

 not being specifically different from a well-known British 

 species, although it was a matter of some difficulty to satisfy 

 myself on this point. My crag specimens vary much, as do 

 also the recent shells : a young one of the latter in my pos- 

 session, corresponds with the fossil in all respects except in 

 the direction of the teeth, which, in the recent, are vertical, 

 while in the fossil, those ^.-laced towards the extremity of the 

 hinge-line are much inclined : I believe, however, that in the 

 recent shell this character may be occasionally observed. — 

 The longitudinal rays, in some specimens, are regular and 

 single, in others they are alternately large and small. The 

 lines of growth are very distinct ; the shell is thickened by 

 the lateral muscular impressions ; the chevron-formed lines 

 in the ligamental area are about foui or five. Small specimens 

 Vol. IV.— No. 41. n. s. 2 e 



