496 GEOLOGICAL SURVEY OF IOWA. 



The description above applies to the Iowa specimens; while those of New-York, 

 which are regarded as identical, present some differences. The shell in the latter is 

 usually stronger, and more convex on the ventral valve, while the dorsal valve is 

 often nearly flat in its upper half, and bent upwards towards the margin : the area 

 is likewise broader, being nearly double the width; the cardinal extremities less 

 salient, and the stride more subdivided and finer in the middle and lower parts of 

 the shell. 



Having no specimens from Iowa which show the interior structure, I have illu- 

 strated these parts from specimens from the Hamilton group of New- York. 



Fig. 5 a, b. Ventral and dorsal views of a specimen of ordinary form and size, from Iowa. 

 Fig. 5 c. A specimen where the cardinal extremities are more salient. 

 Fig. 5 d. Profile showing the convexity of the specimen. 



Fig. 5 e. Interior of the ventral valve of a specimen from New-York, showing the bilobed 

 cardinal process in the centre of the area, and the muscular and vascular 

 impressions. 



Fig. 5/. Enlargement of the area surface, showing the deeply striate character. 



Fig. 6 g. Interior of the dorsal valve of a specimen from New-York, showing, the mus- 

 cular impressions and the double cardinal process. 



Fig. 5 h. The same enlarged, showing the crenulate dental lamella on each side of the 

 base of the cardinal process. 



Fig. 5 i. Enlargement of a portion of the area and cardinal process of the dorsal valve, 

 looking from the outside. In this individual there are no prominent crenulated 

 lamellae, as in fig. 5 h. 



Fig. 5 /c. A similar view of another individual, showing, at the base of the cardinal pro- 

 cess, the indentations made by the two lobes of the process from the ventral 

 valve, and the lateral crenulated lamella?. 



Geological formation and localities. In calcareous shales of the age of the 

 Hamilton group of New- York : New-Buffalo, Pine creek, Independence, 

 Lime creek, and other places in Iowa; Rock-island (Illinois), and in the 

 shales of the Hamilton group in Erie county, New- York. 



Strophodonta fragilis. 



PLATE III. FIG. 6 a, b, c. 



Strophodonta fragilis : HALL, Report of the Regents of the University of New-York, on 

 the State Collections of Natural History, for 1856, p. 143. 



SHELL thin, fragile, semielliptical, usually wider than high; 

 hinge line extending beyond the width of the shell below, 

 and the margins often abruptly contracted just beneath the 

 cardinal extremities. Ventral valve very slightly convex, or 

 nearly flat. Dorsal valve flat or scarcely concave : area entire, 

 very narrow, linear, almost entirely confined to the ventral 

 valve, striated vertically, and crenulated on the inner mar- 

 gins throughout its entire length. 



