CRENELLA. 207 



that portion of the shell a lobated appearance. The anterior 

 half of the shell is much narrower than the posterior, — the 

 shell being regularly attenuated to its front extremity, 

 which is extremely narrow, well and not obliquely rounded, 

 and distinctly projecting beyond the umbones. The 

 straightish and ascending hinder dorsal edge, having united 

 with the posterior one without angularity, runs in a gentle 

 arcuated sweep to the posterior extremity, and thence sud- 

 denly descending, forms a broad and rounded termination 

 to the shell. The umbones are but little swollen ; the in- 

 terior is brilliantly nacreous, with the margin distinctly 

 crenated at the bases of the ribs. 



The very beautiful example which we have figured from 

 Mr. Jeffreys's unrivalled collection, measures half an inch in 

 length, and rather more than a quarter of an inch in 

 breadth. These dimensions exceed those assigned to the 

 Mediterranean examples by Dr. Philippi, from whom we 

 have received small specimens, that perfectly agree with our 

 own in contour, sculpture, and painting. The ribs are not 

 very distinctly developed in the younger shells (nor upon 

 the umbonal region of the adult) ; neither at that stage 

 does the epidermis display the verdant hue which so orna- 

 ments the surface of the mature shell. 



It has been taken at Exmouth by Mr. Clark, in Corn- 

 wall by Mr. Alder and Miss Lavars, and at low-water 

 mark in Oxwich Bay, near Swansea, by Mr. Jeffreys. 



