PALEONTOLOGY OF IOWA. 601 



I have identified this species from several specimens of the ventral valve, which, 

 from having been worn before imbedding in the rock, have the finer surface markings 

 removed. The description given by Messrs. NORWOOD and PRATTEN" of the dorsal 

 [ventral] valve corresponds so precisely with the ventral valve of S. biplicatus, that 

 a doubt may be suggested as to the specific difference of that species. All the spe- 

 cimens of $. biplicatus which I have seen are much smaller than S. forbesi ; the 

 specimen of which, figured on Plate xiii, is about two and a quarter inches in width 

 and an inch in length. 



The S. biplicatus occurs in the yellow sandstone and in the oolitic limestone of 

 the age of the Chemung group, while the S. forbesi occurs in the Burlington lime- 

 stone. It is perhaps possible that the small dorsal valve figured by Messrs. PRATTEN" 

 and NORWOOD may have been derived from a lower horizon than the larger ventral 

 valve. 



Fig. 1. A ventral valve from which the surface markings have been worn. 



Geological formation and, locality. In the Burlington limestone : Bur- 

 lington, Iowa. 



Spirifer imbrex (n.s.). 



PLATE XIII. Fm.2. 

 Compare Spirifer forbesi ubi supra. 



SHELL semielliptical, width about twice as great as the 

 length ; cardinal extremities somewhat rounded, or, in the 

 young state, abruptly pointed : hinge-line equalling the 

 greatest width of the shell. Dorsal valve moderately convex, 

 flattened at the margins and having the mesial elevation not 

 distinctly defined, spreading at the base, covered by plica- 

 tions similar to those on other parts of the valve, of which 

 there are eight or nine proceeding from the dichotomizing 

 of two at the apex : plications on the sides of the valve 

 abruptly rounded or subangular, dichotomizing near the beak 

 and sometimes midway on the valve, becoming about twenty- 

 four on each side at the margin of the shell. 



SURFACE concentrically marked by strong lamellose strise, 

 which are abruptly arched upon the plications ; and towards 

 the margin of the shell, by a few strong imbricating lines 

 of growth. 



This species resembles the . forbcsii ; but there are a greater number of plications 

 on the mesial fold, and those of the lateral portion of the shell are dichotomized, 



[ IOWA SURVEY-] 76 



