GEOLOGICAL SURVEY OF IOWA. 



of the shell slightly arching over the area ; mesial fold 

 broadly rounded, moderately elevated, and defined on each 

 side by a groove broader than those between the plications. 

 SURFACE marked by from four to six ( often three in young 

 specimens) rounded and very moderately elevated plications 

 which are separated by shallow depressions between, the 

 whole becoming obsolete before reaching the beak and hinge 

 line ; concentrically marked by fine closely arranged striae 

 and more distant imbricating lamellse, upon the edges of 

 which are arranged, in regular concentric undulating lines, 

 short longitudinal elevated striaa, or more properly the elon- 

 gated bases of minute semitubular spines, which ornament 

 the lower edges of each successive imbrication, and are 

 formed by the contraction and separation of the extended 

 edges of the lamellae. 



This is the only species to which I can refer the description given by Mr. CONRAD 

 of Delthyris jimbriata. The species occurs in the Upper Helderberg limestone and in 

 the Hamilton group of New- York; and I have seen a specimen of, apparently, the 

 same species in the Oriskany sandstone. The western specimens are usually smaller 

 than those of New- York ; though the one figured by Dr. OWEN, pi. iii, f. 7, is equal 

 in size to the larger specimens observed in any localities. 



There are rarely six, and still more rarely seven, defined plications upon each side 

 of the mesial fold and sinus : commonly there are no more than four, sometimes 

 two or three, and in one specimen of more than half- an inch in diameter there are 

 no defined plications. In rare examples the plications are defined nearly or quite to 

 the beak. The short semitubular spines are rarely preserved, though the bases are 

 well marked. In the absence of external characters, the elliptical form of the shell, 

 with the broad round plications, even in casts, will generally serve to distinguish 

 the species. 

 Fig. 5 a. A fragment of a ventral valve, in which the sinus is subangular. 



Fig. 5 b. Cardinal view of the same, showing the broad foramen which occnpies a large 

 part of the area. 



Fig. 5 c. Dorsal valve of a small individual, preserving the spiniferous bases. 

 Fig. 5 d. Profile view of a small specimen from the Hamilton group of New-York. 



Fig. 5 e. Enlargement of the surface as it usually appears when the bases of the spines 

 are preserved. 



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

 Hamilton group of New- York : Davenport, New-Buffalo ( Iowa ); Rock- 

 island ( Illinois ). 



