294 



Original description of Pentamerus decussatus. " Shell large, usually 

 longitudinally and rather narrowly subovate, about one third longer than 

 broad, and broadest a little in advance of the mid-length, but sometimes 

 nearly as broad as long ; front margin regularly rounded in most speci- 

 mens, 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 longitudinal 

 depression in the centre, its beak small, rather narrow and slightly 

 incurved. 



" Surface marked by very numerous, closely disposed, rounded and but 

 slightly elevated radiating raised lines, which are crossed by smallier, 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, comparatively 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 deve- 

 loped and almost rudimentary, very slightly divergent and much shorter 

 than the ventral septum. Muscular and vascular 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-two mm.; amount of recurvature of beak 

 of ventral valve, sixteen mm. 



" The only locality in which this species is known to the writer to have 

 been certainly found in place, is in a light brownish yellow dolojnitic lime- 

 stone at the foot of the Grand Rapids of the Saskatchewan, where a number 

 of fine specimens were collected by Mr. Tyrrell in 1890. Boulders con- 

 taining it have oeen found at several localities in Manitoba, and elsewhere 

 in the central portion of the Dominion. It is almost certainly the shell 

 referred to by Sir John Richardson as a ' Pentamerus, very like P. 



