114 



Contributions to Palceontology . 



wide : area, high, with beak much advanced ; cardinal 

 margins sloping backwards in a straight line to the 

 extremities. 



Surface strongly striated: strise fasciculate, with a few 

 concentric lines of growth. The cast of the ventral 

 valve shows a strongly striated area, a large and long 

 foramen, with the central portion markedby a few strong 

 simple vascular impressions. 



The area appears to have been on the same plane with 

 the margins of the shell, and, in the casts, does not always 

 show the strong transverse strise. The cast of the dorsal 

 valve shows very little of important features : it is regu- 

 larly convex, and a little flattened in the middle. The 

 cardinal process has been very thin and small, with marks 

 of strong socket-plates : from the outer extremities of these 

 proceed an impressed line which curves towards the sides, 

 and is directed forwards, enclosing a space which is proba- 

 bly the muscular area. 



Compared with Orthis color dtdensis, in specimens from Texas, 

 received from Dr. Francis Moore, this species is usually smaller, 

 the length of ventral valve proportionally greater, with a higher 

 area and finer striae. 

 Fig. 23. Dorsal valve of Orthis pepina. 



Fig. 24. Ventral valve of same species, the form more than usually elon- 

 gated. 



Fig. 25. Cast of dorsal valve of same. The line limiting the muscular 



impression is made too strong, and the elevation of the central 



portions too abrupt. 

 Fig. 26. Cast of ventral valve, showing height of area, cast of rostral 



cavity, and vascular impressions. 

 Fig. 27. A cast of the ventral valve, showing a narrower area, with the 



cavities made by the dental lamellae. 



The specimen of the cast of the dorsal valve figured is the largest that 

 has been found. 



The Orthis pcpi?ia, though occurring in several localities, has 

 been obtained in characteristic specimens only in yellowish and 

 sometimes light buff-colored sandstone on Lake Pepin, above Reed's 

 landing. It occurs at Miniska on the Mississippi river, and at 

 Osceola on the St. Croix river. 



