CRENELLA. 209 



fringed, as it were, by rather remote and very delicate 

 concentric laminae, that are chiefly apparent on, if not in- 

 deed confined to, the hinder portion of the shell, and when 

 worn off, indicate their former presence by a kind of inter- 

 rupted or obsoletely articulated appearance which they be- 

 stow upon the radiating striae. The ventral margin is retuse 

 in the middle, and convex at the extremities ; it ascends more 

 rapidly, but still arcuatedly, in front. The hinder dorsal 

 edge, which, although constantly elevated, varies much in 

 height, rendering the shell more elongated when the dorsal 

 angle is less developed, more abbreviated when the rise is 

 more manifest, forms a rounded off but distinct angle with 

 the posterior margin ; the latter, after arching out, be- 

 comes incurved a little below the middle of the hinder side, 

 thus giving a somewhat lobated appearance to the narrow 

 and well-rounded tip of the posterior side. The beaks are 

 prominent, and being terminal, project beyond the anterior 

 end, which is not at all angulated, but is arcuatedly cut 

 off below, so that the lower portion is the more receding. 

 The largest recorded example is stated by Capt. Brown to 

 have measured a quarter of an inch in length, and an 

 eighth of an inch in breadth ; such dimensions are, how- 

 ever very uncommon, the average length not exceeding a 

 fifth of an inch. Some of the best specimens as yet found 

 are the two preserved in the National Museum, and j^re- 

 sented respectively by Mr. Prideaux, who obtained his 

 shell on the Devonshire coast, and the Rev. Miles Berkeley, 

 who dredged a single individual at Weymouth adhering 

 by its byssus to a large mass of slate. Odd valves have 

 occurred to us in shell sand from Herm (S. H.) ; Mr. 

 M 'Andrew dredged it in 20 fathoms off Penzance, and 

 Mr. Barlee off Brora Island. 



VOL. II. E E 



