508 GEOLOGICAL SURVEY OF IOWA. 



Spirifer aspera ( n. s.). 



PLATE IV. FIG. 7 a, b, c, d. 



SHELL semielliptical, width about twice the length ; hinge 

 line equalling the greatest width of the shell, and abruptly 

 angular at the extremities. Ventral valve triangular pyra- 

 midal, extremely elevated at the umbo, and sloping abruptly 

 to the lateral and anterior margins ; mesial sinus shallow, 

 strongly denned at the edges, sides straight, flattened with- 

 in, sometimes curvilinear : area straight and flat, abruptly 

 defined at its margins, equal or less in height than the length 

 of the valve, transversely and vertically striate ; foramen 

 proportionally narrow, extending to the beak, and partially 

 closed within by a pseudo-deltidium ; beak not incurved. 

 Dorsal valve nearly flat or moderately convex ; mesial fold 

 strongly defined at the margins, and rounded above. 



SURFACE, in full-grown specimens, marked by twenty or 

 more simple rounded and moderately elevated plications on 

 each side of the mesial fold and sinus. Plications often fifteen 

 to eighteen in ordinary specimens, four or five only of which 

 reach the beak, the others running out along the hinge line 

 and margins of the area ; concentrically marked by fine 

 imbricating lines of growth, which are sometimes very con- 

 spicuous. Entire surface granulose ; granules closely arranged, 

 sometimes visibly following the concentric strise, and in other 

 parts apparently scattered without order. 



This species approaches in form the S. macronotus of the Hamilton group of New- 

 York, and may be regarded as its western representative. This one differs in its less 

 cardinal extension, more abrupt elevation of the ventral valve and proportionally 

 higher area, fewer plications and granulose surface. It bears a close resemblance in 

 form to the S. tenticulum ( MURCHISON and DB VERNEUIL, Russia and the Ural 

 Mountains, pa. 159, pl.v, f. 7); but the sinus of that species is plicated. 

 Fig. 7 a. Ventral valve of a specimen less than the full size. 

 Fig. 7 b. Cardinal view of the same, showing the area and foramen, which is partially 



filled with the pseudo-deltidium. 

 Fig. 7 c. Dorsal valve of a larger individual, which shows about twenty strue on each 



side of the mesial fold. 



Fig. 7 d. Enlargement of the surface, showing the granulose character, with a few dis- 

 tinct imbricating lines. 



