174 PALEONTOLOGY. 



and various fresh- and some brackisli- water shells, which occur there Ly 

 millions. Dr. Hayden's and Mr. King's parties have brought large collec- 

 tions of these shells from this locality ; and Mr. Durkee, an intelligent 

 civil engineer, sent great quantities of them to the Smithsonian Institution. 

 I have, as elsewhere stated, referred these beds to the Lower Eocene ; but 

 they may yet prove rather to belong to the latest Cretaceous, as suggested 

 by me in Dr. Hayden's Reports, and in the Upper Missouri Palaeontology. 



OoRBULA ( Anisorhynohus *?) Engelmanni, Meek. 



Plate 17, figs. 1 and 1 a. 



Corhula [Anisorliynchus ?) Engelmanni, Meek (1860), Proceed. Acad. Nat. Sci. Philad., 

 XII, 312. 



Shell subovate, moderately gibbous, nearly equivalve; anterior margin 

 rather narrowly rounded; base forming a semi-ovate curve, with the most 

 prominent part in advance of the middle; posterior more or less narrowed 

 and compressed, with the immediate extremity slightly truncated; beaks 

 rather depressed, nearly or quite equal, and placed a little in advance of the 

 middle; dorsal outline sloping gradually before and behind the beaks 

 toward the extremities, with a shallow marginal furrow extending from the 

 beaks posteriorly; lunule excavated, but not sharply defined. Surface with 

 small, more or less regular, concentric furrows and strise. 



Length of the typical specimen, 0.50 inch; height, 0.20 inch; convexity, 

 0.20 inch. 



Since seeing how greatly the preceding species varies in form and 

 other characters, I am led to suspect that this may be only a 3'oung or 

 more depressed form of the same shell. Its greatest differences, aside from 

 its generally smaller size, are its less gibbous form, more depressed beaks, 

 placed a little farther backward, and its proportionally less attenuated pos- 

 terior, with its dorsal margin merely sloping backward from the beaks with- 

 out being concave in outline. Although with my present impressions of 

 the variable character of C. pyriformis, I should hesitate to separate the 

 form under consideration from that species, if I were now for the first time 

 investigating these shells, as it has already been described under another 

 name, it may be better to keep them separated until we can have an oppor-' 



