CYRENA. 31 



A very rare species, tlie only specimens I have seen being the 

 one in Mr. Cuming s cabinet and a young one in my own, which 

 was received from him. It is easily distinguished from all others 

 by the peculiar outline of the &quot;posterior side which terminates in 

 a very acute angle. The shell marked C. perumana in Mr. 

 Cuming s collection, from which Mr. Deshayes described it, 

 belongs without doubt to the species above. 



FOSSIL SPECIES. 



35. Cyrena densata, CONRAD. &quot;Shell subtriangular, thick, con 

 vex ; anterior margin obtusely rounded ; basal margin profoundly and 

 regularly curved to the posterior extremity, which is subtruncated, direct, 

 and greatly above the line of the base ; beaks central, summits elevated ; 

 strise robust ; teeth large, robust, very prominent ; middle tooth of the 

 right valve bifid ; lateral teeth elongated, robust, anterior tooth truncated, 

 suddenly deflected at the extremity, posterior tooth distant.&quot; 



Long. 1.87; Lat. 1.80 inches. 4 



Hob. North America, at Petersburg, in the State of Virginia. Tertiary 

 formation. (Cabinet of ?.) 



Cyrena densata, CONRAD, Proc. Ac. N. S. Phil. 1, 1845, 324. 



36. Cyrena dakoteiisis, MEEK and HATDEN. &quot; Shell suborbicu- 

 lar, or broad ovate-subtrigonal, moderately convex ; anterior and posterior 

 sides rather abruptly rounded ; base forming a semi-oval curve ; dorsal 

 outline sloping from the beaks, the anterior slope being a little concave, 

 and the posterior convex in outline ; beaks rather elevated and subcentral ; 

 anterior muscular impression narrow, ovate, well defined ; posterior broader 

 and more shallow : palleal line distinct, nearly simple, or very faintly 

 sinuous j ust beneath the posterior muscular scar ; surface marked by more 

 or less distinct concentric strise.&quot; 



Long. 1.20 ; Lat. 1 ; Diam. 0.58 inches. 



Hdb. North America, at the mouth of the Big Sioux or Dakotah River. 

 Dakotah group of the Nebraska and Dakotah cretaceous series. (Cabinet 

 of the Smithsonian Institution.) 



Cyprina arenaria, MEEK & HAYDEN, Proc. Ac. N. S. Phil. 1857, 143. 



