492 GEOLOGICAL SURVEY OF IOWA. 



valve, receiving between its forks the cardinal process of the 

 opposite or ventral valve, which is bilobed or grooved for 

 the passage of the peduncle. Entire interior surface papillose. 



The species of this genus have the general form of STROPHOMENA or LEPTJENA., 

 and are similarly striated, in all the variety of simple, bifurcating and fasciculate 

 striae. The area, which is common to both valves, is usually entire, in the species 

 known, with the margins distinctly crenulated. There is sometimes a linear callosity 

 along the centre of the area of each valve, in the place of the triangular foramen ; 

 and in some specimens of L. demissa, there is a triangular space on the area of each 

 valve, but which is solid like the other parts, and to which are affixed the cardinal 

 processes. In some species there is sometimes a small semicircular opening at the 

 base of the area of the ventral valve; and in one individual of another species, 

 I have seen a triangular foramen with a round opening between the forks of 

 the cardinal process of the dorsal valve, while the cardinal process of the ventral 

 valve projected downwards, interlocking with the opposite one, and forming one side 

 of the circular foramen. 



The muscular and vascular impressions of true STROPHODONTA are in general more 

 like those of CHONETES or PRODUCTUS ; and are for the most part distinct from 

 STKOPHOMENA ; while the crenulated margins and absence of triangular foramen, 

 character of the cardinal processes, etc. furnish ready means of distinction. 



Strop! to dont a arcuata (n. s.). 



PLATE III. FIG. 1 a, b, c, and FIG. 2 a, b, e,f 



SHELL semielliptical ; hinge line scarcely equalling the 

 greatest width of the shell near the cardinal extremities. 

 Ventral valve hemispheric, gibbous in the middle, sometimes 

 with an obtuse undefined elevation along the centre, the 

 cardinal extremities usually a little recurved. Dorsal valve 

 deeply concave ; area common to both valves, that of the 

 ventral valve larger and deeply striated vertically : foramen 

 none, or rarely an opening at the base of the area. 



SURFACE marked by fascicles of strong striae, or which 

 sometimes consist of a few strong ones near the beak, bifur- 

 cating below, while the general character of surface marking 

 is that of strong, sharply elevated striae, with three, four or 

 five smaller ones between : worn surface punctate or striato- 

 punctate. 



Interior of shell papillose. In the ventral valve the im- 

 pressions of the adductor muscles are small but well defined, 



