INVERTEBRATE PALAEONTOLOGY. 159 



Cyrena Dakotcnsis, M & H. 



Plate ] , figs. 1, a, ft, e, d, e, f. 



Cyprina arenaria, Meek and Hayden (1857), Proceed. Acad. Nat. Sci. Philad., 143. 



Cyrena arenaria, Meek (1864), Smithsonian Check-List Cret. Moll., 13 (not '.'. arenaria, Forbes). 



Cyrena Dakotensis (Meek and Hayden, MS ). — Prime (1865), Monogr. Am. Corbiculidw, 31. 



Shell subcirctilar, or very broad-subovate, moderately convex; anterior 

 and posterior margins rather abruptly rounded ; base forming a semi-oval 

 curve; dorsal outline sloping from the beaks, the anterior slope being abrupt, 

 a little concave, and the posterior convex. Beaks rather elevated, and sub- 

 central ; anterior muscular impression narrow-ovate, well defined ; posterior 

 broader and more shallow. Pallial line distinct, nearly simple, or very faintly 

 sinuous just beneath the posterior muscular scar. Surface marked by more 

 or less distinct concentric striae. 



Length, 1.20 inches; height, 1 inch ; convexity, about 0.58 inch. 



I have seen no very good specimens of this species ; all of those yet 

 found being casts of the exterior and interior. In first describing it, we 

 referred it to the genus Cyprina, mainly from its form and general appear- 

 ance. Some casts of the interior subsequently obtained, however, show that 

 its hinge-characters, so far as they can be made out, differ from those of 

 Cyprina, and agree much more nearly with those of Cyrena; and that its 

 pallial line is also faintly sinuous. Consequently, I referred it to Cyrena in 

 the Smithsonian Check-List; but, in making this change, inadvertently over- 

 looked the fact that Professor Forbes had previously used the specific name 

 arenaria for a species of that genus. Subsequently, however, I noticed that 

 this name had been previously used in the genus Cyrena, and adopted the 

 name C. Dakotensis for our shell, in manuscript ; and on informing Mr. Prime 

 of this fact, he very properly inserted the description of the species under 

 that name in his valuable Monograph of the Corbiculidce. 



It is possible that, when better specimens can be found, this shell 

 may prove to belong to some other genus, as it occurs associated with the 

 marine types Axincea and Mactra. Still, we also find, in the same associa- 

 tion, a species having all the external characters of the estuary genus 

 Pharella; while the rock is a coarse sandstone, containing many leaves of. 

 trees, some silicified wood, &c, just as we might expect would occur in a 



