PALEONTOLOGY OF IOWA. 491 



SURF A OP marked by fine subequal elevated striae, which 

 are crossed by very fine well defined concentric striae and 

 some imbricating lines of growth. 



This species is similar to Orthis perversa of the Hamilton group of New- York, 

 but differs in the arrangement and character of the striae. 



A single ventral valve only has been observed, and it is noticed here to call atten- 

 tion to its occurrence in Iowa. 



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

 Hamilton group : Lime creek, Worth county, Iowa. 



GENUS STROPHODONTA. 



Strophodonta : HALL, Palaeontology of New-York, Vol. ii, p. 63. 



SHELL depressed expanded, semielliptical, semicircular 

 or subquadrate, transverse or elongated (external surface 

 striated ) ; hinge line straight, usually as long as the width 

 of the shell : ventral valve convex or concave ; dorsal valve 

 following the curves of the opposite, often abruptly bent 

 downwards along the middle or towards the margin ; area 

 more or less common to both valves, more developed on the 

 ventral valve, crenulated on the inner edges ; fissure none 

 or partial, a linear callosity sometimes marking the centre 

 of both areas. 



In the ventral valve the teeth are much reduced or nearly 

 obsolete, a central more or less prominent bilobed process 

 usually occupying the centre of the area in place of the 

 triangular fissure of STROPHOMENA. Muscular impressions 

 strongly marked, semielliptical or subreniform, separated in 

 the middle by a depressed line, and sometimes margined by 

 a semicircular ridge, which is an extension of the lamellae 

 from either side. Vascular impressions foliate or flabellate, 

 extending beyond the areas towards the base of the shell. 



Dorsal valve with the muscular and vascular impressions 

 strongly marked : cardinal process bifurcate from the base, 

 with each branch bilobed at the extremity by which it is 

 articulated to processes beneath the area of the opposite 



