144 GASTROCIIiENID.E. 



ligament is small, sunken, and of a yellowish brown ; the 

 beaks sharply defined, acute, rather prominent, and leaning 

 forward ; having in front of them a deeply impressed, more 

 or less ovate lunule. The interior is of a pure glossy white 

 (sometimes subnacreous beneath the umbones), the margin 

 entire, and the hinge, when not entirely obliterated with 

 age (in which case the margin itself displays a considerable 

 callosity), consisting of a single strong acute primary tooth 

 in the right valve, interlocking between a rather smaller 

 and a perfectly rudimentary one in the left valve. This 

 latter minute denticle is sometimes present, but more usu- 

 ally absent, in both valves. The convexity of the valves 

 sometimes amounts to ventricosity, more ordinarily they 

 are but moderately convex, but there is always an appear- 

 ance of compression upon the umbonal region, owing to the 

 constant concavity of that portion of the surface which 

 precedes the diagonal elevation. 



The size of our British specimens is greatly inferior to 

 that of foreign examples, and almost always with us is less 

 than in the succeeding species, the average of individuals 

 not exceeding two-thirds of an inch in length, and two- 

 fifths of an inch, at most, in breadth. 



The animal of this species is oblong or cylindrical, its 

 mantle closed in front except a small orifice for the passage 

 of the foot, which is very small and conical, and furnished 

 with a byssal groove. The siphonal tubes are short, nearly 

 equal, and united very nearly to their extremities, which 

 are each furnished with about ten or twelve cirrhi. The 

 body and foot are usually white or yellowish, the tubes 

 orange, rose-colour, or brownish, varying much in intensity 

 of colour. 



The Iliatella arctica, though distributed throughout the 

 Ikitish seas, is far more abundant in the north than in the 



