81 



very oblique, located at the anterior extremity, and pro- 

 jecting a little above and beyond the termination of the 

 hinge. Hinge line straight, nearly equaling the greatest 

 length of the shell; escutcheon narrow and long. Surface 

 showing rather obscure traces of concentric marks of 

 growth, which are crossed, on the postero-dorsal region 

 of each valve, by two or three small indistinct radiating 

 lines or narrow ridges, extending from the beaks towards 

 the posterior margin. Length 0*37 inch \ heigth 0*16 inch ; 

 breadth 0*14 inch. 



Not having seen the hinge or interior of this shell, we 

 refer it only provisionally to the above genus, all its exter- 

 nal characters being very similar to P. costatus Brown, as 

 figured by Mr. King in his work on the Permian Fossils of 

 England. 



Locality. — On the Missouri, in Nebraska, opposite the 

 northern boundary line of Missouri. 



Pleurophorus (Cardinia) subcuneata. 



Shell elongate, moderately compressed, most convex from 

 the beaks obliquely backwards towards the lower part of 

 the posterior end ; extremities narrowly rounded, the more 

 prominent part of each being below the middle. Dorsal 

 margin straight, and nearly horizontal, or declining slightly 

 from the beaks towards the anal end ; basal margin nearly 

 straight, sometimes a little concave, but often slightly con- 

 vex in outline. Hinge long, straight, and. apparently pro- 

 vided with one long linear posterior lateral tooth in the 

 right valve, which fits into a corresponding groove in the 

 edge of the other ; cardinal te«th unknown ; escutcheon 

 narrow. Beaks small, oblique,, little elevated above the- 

 hinge, and located near the anterior end.. Length 0*60 inch ^ 

 height 0-29 inch ; breadth 0*18 inch. 



The posterior muscular impression is very faint, compa- 

 ratively large, subquadrate, and placed neas the posterior 

 end of the hinge. The anterior muscular impression is- 



[Trans, iv.] 12 



