CONTRIBUTIONS TO PALAEONTOLOGY. 135 



Shell semielliptical. Dorsal valve convex, about three-fourths as 

 long as wide, sometimes proportionally narrower : hinge-line as 

 long as the greatest width of the shell. Ventral valve a little 

 more convex than the dorsal, longer than wide, sometimes one- 

 fourth longer than wide,-:' area high, with beak much advanced ; 

 cardinal margins sloping backwards in a straight line to the 

 extremities. 



Surface strongly striated : striss fasciculate, with a few concen- 

 tric lines of growth. The cast of the ventral valve shows a 

 strongly striated area, a large and long foramen, with the 

 central portion marked by a few strong simple vascular im- 

 pressions. 



The area appears to have been on the same plane with the 

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

 strong transverse stride. The cast of the dorsal valve shows very 

 little of important features : it is regularly 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 to- 

 w^ards the sides, and is directed forwards, enclosing a space which 

 is probably the muscular area. 



Compared with Orthis color adensis, 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 elongated. 



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

 made too strong, and tho 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 pepina, though occurring in several localities, has been 

 obtained in characteristic specimens only in yellowish and sometimes light 

 huff 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. 



