604 GEOLOGICAL SURVEY OF IOWA. 



Spirifer grimesi ( n. s.). 



PLATE XIV. Fm.l -5. 



SHELL very large, gibbous, varying from longitudinally to 

 transversely subelliptical : hinge-line usually less than the 

 greatest breadth of the shell, obtusely angular or a little 

 rounded at the extremities. Dorsal valve very convex, gib- 

 bous, rising in the middle into a broad subangular undefined 

 mesial fold, which becomes much expanded towards the base. 

 Ventral valve convex : mesial sinus rather broad, subangular 

 below, not distinctly defined at the margins ; area of mode- 

 rate height, short ; foramen broad triangular ; beak strongly 

 incurved. 



SURFACE ornamented by numerous depressed rounded ir- 

 regularly bifurcating plications, about twenty to twenty-five 

 of which occupy the mesial fold and sinus : plications marked 

 by closely arranged extremely fine longitudinal striaB, and 

 equally fine concentric striaB, with some stronger imbricating 

 lines of growth. 



Interior of ventral valve showing a broad suboval muscular 

 area, limited by the dental Iamella3 which diverge from the 

 base of the foramen, and again converge in a low ridge, 

 approaching each other in the middle or towards the front 

 of the valve. 



Nearly all the specimens which I have seen of this fine spirifer are more or less 

 distorted, and give for the most part an imperfect idea of its form. It occurs gene- 

 rally in the condition of separated valves, which have apparently a longer cardinal 

 line than is shown in the only entire individuals seen. It is, however, clearly distinct 

 from S. striatus and S. sowerbyl, to which it has been referred. From the former it 

 may be distinguished by its less transverse form, short hinge, broader and more 

 shallow and subangular mesial sinus, as well as by the peculiar mode of bifurcation 

 of the plications, by which they are separated into pairs, and never increase by 

 implantation. 



In its general external features it resembles <S. sowerbyi ; but it has not the strong 

 parallel dental laminae, which, in that species, extend from the base of the foramen 

 nearly the entire length of the shell, while the form of the area and other characters 

 are equally distinctive. It is not improbable that the imperfect ventral valves of <S. 



