132 CYCLADIDJi. 



convex. The posterior end is of the ordinary shape in this 

 genus, — that is to say, bhint, rounded below, and subangu- 

 lated above ; the hinder margin itself is not greatly convex ; 

 the dorsal edge is straightish and rather sloping. The 

 chief profundity is decidedly at the umbonal region, the 

 anterior end being manifestly compressed, and the valves 

 being generally more or less pinched in on either side of the 

 umbones. These last are rather acute, and are furnished 

 Avith a small laminar eave-like projection, which in the 

 adult occupies quite the summit of the shell, appearing 

 like a ridge i-ising nj) vertically on either side of the hinge, 

 but in the young looks like a small wing proceeding from 

 the middle of the shell. 



Specimens which measured two lines and a half in length, 

 and two lines in breadth, are recorded by Mr. Jenyns, 

 — dimensions far exceeding the average of our examples. 



The animal, according to Jenyns, is white, with a short, 

 generally subconic, siphonal tube. 



This is a rare shell, which was first discovered by Pro- 

 fessor Henslow in ditches communicating with the river 

 Cam, in the immediate neighbourhood, and a few miles 

 below Cambridge (Jenyns). It is likewise met with in 

 Suffolk (Sheppard) ; in the Thames at Henley (Strick- 

 land), at Cadley, near Swansea (Jeffreys), and at Finnoe 

 in Tipperary (Thomp. Ann. N. H. 6. p. 196). 



It has been noticed in Germany. As a fossil, it is found 

 in the freshwater pleistocene beds of Grays and elsewhere 

 in the south of England. 



