23a 



The Ottawa Naturalist. [March 



rounded below. Ventral margin broadly rounded in front of the 

 midlength and straighter behind it ; superior border sloping 

 rapidly, obliquely, and somewhat concavely downward in fro t of 

 the beaks, and more gradually and convexly so behind them ; 

 umbones rather narrow and moderately prominent ; beaks curved 

 inward and forward, placed in advance of the midlength. 



Surface marked with numerous concentric lines of growth ; 

 test thick and apparently not nacreous. 



Hinge dei^tition and muscular impressions unknown. 



Approximate dimensions of the specimen described and 

 figured, which has both valves preserved though slightly dis- 

 placed : maximum length, 72 mm. ; height, 64 mm. ; greatest 

 thickness or convexity of the two valves when closed, about 

 41 mm. 



Judging by its external characters and by analogy with other 

 fossil and recent species, this specimen would seem to be referable 

 to Cyrena rather than to (^orbicula. If it is a true Cyrena, it is 

 the first species of that genus that has been recognized in Canada. 

 But according to Mr. Meek, " it is quite probable that a critical 

 study of the numerous extinct species that have been described 

 under the general name Cyrena^ by those who are not very parti- 

 cular in regard to generic distinctions, would bring to light suffi- 

 cient reasons for the separation from that genus and Corbieula, of 

 several groups, either holding the rank of distinct genera or sub- 

 genera."* And, in reference to Cyrena, Mr. Meek had pre- 

 viously remarked that " Mr. Prime has called attention to the 

 fact that, in the existing American species of this genus and Cor- 

 hicula, the pallial line is always distinctly sinuous, the sinus being 

 comparatively deep and sharply angular ; while in foreign species 

 it is but slightly, or sometimes not at all, sinuous." " So far as I 

 have had an opportunity to determine," Mr. Meek adds, " all of our 

 far-western fossil species, excepting C. Dakotetisis, have a more or 

 less distinctly sinuous pallial line ; but, in no instance have I seen 

 the sinus so deep and sharply angular in the latter, as represented 

 by Mr. Prime in some of the recent species."! In a future re- 



* Report of the U. S. Geolog^ical Survey of the Territories, vol. ix, p. 160. 

 t Idem, p. 157. 



