DIPLODONTA. 67. 



ascending ventral ; this latter is not symmetrically arcu- 

 ated, but in the adult rises principally and obliquely in front, 

 is not very convex beneath the beaks, and curves out chiefly 

 towards its posterior end. The hinder side is much the 

 broader, its upper corner is more or less distinctly angu- 

 lated, and its lower rounded off; the posterior margin is 

 usually more or less curved, but varies much in that respect, 

 and the upper or dorsal margin is straight, produced, and 

 not at all sloping. The umbones are moderately swollen, 

 obliquely inclined, and not particularly prominent ; the 

 beaks, which are small and inflected, are neither preceded by 

 a lunule, nor any peculiar flatness of the dorsal area. The 

 ligament, which, though rather small and much projecting, 

 is distinct, and occasionally of a golden yellow, is not en- 

 vironed by any excavation. The internal edge of the 

 ventral margin is acute, entire, and not particularly flex- 

 uous. There are no lateral teeth, but two rather small 

 primary ones in each valve ; of which the bifid is posterior 

 in the right one, and anterior in the left, where the simple 

 tooth is narrow and very oblique. The muscular impres- 

 sions are large, of rather a produced shape, and nearly 

 equal magnitude ; the pallial impression is broad and 

 shining. 



The largest example we have ever seen, procured by Mr. 

 Alder from the Cornish fishermen, measures exactly one 

 inch in length, and eleven lines in breadth at the widest 

 part. Such dimensions are nearly unparalleled, the ordi- 

 dary diameter being only five-eighths of an inch. 



We have never seen the animal, but during the course 

 of the summer of 1848 it was met with by Mr. Clark, who 

 communicates the following account of it : — " Animal 

 lentiform, moderately thick ; mantle plain, somewhat closed 

 posteriorly and anteriorly, but with a large opening for the 



