296 Canadian Record of Science. 



as broad as long ; front margin regularly rounded in most 

 specimens, but somewhat pointed in the centre in others* 

 Ventral valve strongly convex, very tumid, prominent, and 

 rounded or obtusely angulated along the median line, and 

 narrowing rapidly to the margin on both sides, but devoid 

 of a distinctly defined mesial fold, its umbo prominent and 

 rather broad, and its beak so strongly recurved as almost to 

 touch that of the opposite valve. Fissure rather large, 

 triangular, a little higher than broad, completely covered 

 by the recurved beak and visible only when the beak is 

 broken off. Dorsal valve much flatter than the ventral, 

 gently and uniformly convex, or flattened with a faint longi- 

 tudinal depression in the centre, its beak small, rather nar- 

 row and slightly incurved. 



Surface marked by very numerous, closely disposed, 

 rounded and but slightly elevated radiating raised lines, 

 which are crossed by smaller, more close set and irregularly 

 disposed concentric raised lines, as well as by a few distant 

 and more or less imbricating lines of growth. The radiating 

 raised lines, which are rather irregular in their arrangement 

 and unequal in size, increase so rapidly by division that as 

 many as from sixty to one hundred or more of them can be 

 counted around the front margin of an adult specimen, 

 though, on account of its greater convexity, there is always 

 a larger number on the ventral valve than on the dorsal. 



Septum of the ventral valve well developed, compara- 

 tively thick but very short, occupying less than one fourth 

 of the entire length in some specimens, but a little longer 

 in others, though rarely or never exceeding one third of the 

 total length. Septa of the dorsal valve thin, feebly devel- 

 oped and almost rudimentary, very slightly divergent and 

 much shorter than the ventral septum. Muscular and vas- 

 cular impressions unknown. Interior of the valves rather 

 minutely papillose. 



Dimensions of the specimen figured; maximum length, 

 eighty seven millimetres, greatest breadth, fifty nine mm. ; 

 maximum height or depth through the closed valves, fifty 



