294 



■which is obtuse and incurved; area scarcely perceptible; foramen 

 broad, triangular, nearly as wide at the base as the length of the area ; 

 front half of the valve marked by a broad, shallow, undefined sinus. 

 Dorsal valve less ventricose than the opposite, regularly convex, 

 without a visible mesial elevation ; beak obtuse, incurved, extending 

 above the cardinal line. 



Surface marked by strong, equidistant, concentric ridges, indicating 

 different stages of growth ; also by indistinct, radiating strias, which 

 form little pustules at the margin of the ridges, as if for the attach- 

 ment of seta3. No appearance of plications have been observed on any 

 of several specimens examined. 



This species is closely related to S. pseudoUneatus, Hall (Geol. 

 Ecpt. of Iowa, Vol. I., Part ii., page 645, pi. 20, fig. 4), from which it 

 difiers in being proportionally narrower ; in the shorter hinge line, 

 and smaller area ; and the mesial elevation seldom projects above the 

 general convexity of the valve. From its general form and the closely 

 incurved beaks, as well as from the lamellose surface, it has been 

 mistaken by collectors for an Athyris. 



Genus Retzia, King. 

 Retzia sexplicata (n. sp.) 



Shell small, broadly subtri angular, or subquadrangular, very ventri- 

 cose ; marked by about six strong, simple, deep plications ; the two 

 central ones direct from the beak to the front margin, the others 

 strongly curved outwards ; the outer ones on each side very small, 

 scarcely seen on young shells. Hinge short. Beak of the ventral 

 valve elongate, slender, largely perforate at the extremity, and deeply 

 marked by the plications to the margin of the perforation ; area small, 

 triangular. Dorsal valve less ventricose than the opposite, beak de- 

 pressed, hinge alations minute. 



Surface of the shell marked by very fine concentric strise, which are 

 undulated as they cross the plications, and frequently by strong con- 

 centric ridges, indicating different stages of growth. 



This species very closely resembles Retzia '? ferita, Terehratula ferita, 

 Von Buch, but is rather smaller, with fewer plications, and the area is 

 more distinct. 



Genus Rhynchonella, Fisher. 



Rhynchonella opposita (n. sp.) 



Shell very small, triangular in outhne ; cardinal slopes straight, and 

 strongly divergent, forming an angle of about seventy-five or eighty 

 degrees, equalling in length the breadth of the front. Valves sub- 

 equally convex; the ventral regularly arcuate from beak to base; 

 beak small, incurved, perforate (perhaps from accident). Dorsal valve 



