SPIRIFER 341 



ties where the surface is usually somewhat compressed, especially in those 

 forms with an extended hinge-line, curving more abruptly to the cardinal 

 margins than to the antero-lateral margins, surface of each lateral slope 

 marked by from 9 to 13 simple, rounded plications which grow fainter 

 towards the cardinal extremities, the most usual number being 10 or 11 ; 

 mesial sinus originating at or near the beak, angular and sharply defined 

 at first, becoming rounder and less sharply defined anteriorly where it is 

 rather shallow or of moderate depth ; near the beak a median plication 

 originates in the sinus and continues without division, becoming gradu- 

 ally stronger to the anterior margin ; on each side of the median plication 

 there may be a single plication which is usually, but not always, weaker 

 than the median one, wniefe originates in the bifurcation of the lateral 

 bounding plication of the sinus j occasionally a second plication is present 

 on one or both sides which originates through a second bifurcation of the 

 bounding plication nearer to the anterior margin. 



Brachial valve a little less convex than the pedicle, the greatest convex- 

 ity near the middle, the surface sloping more abruptly to the cardinal 

 margin and becoming somewhat compressed towards the cardinal extrem- 

 ities ; mesial fold sharply defined to the beak, at first scarcely, or not 

 at all, elevated above the general surface, becoming gradually elevated an- 

 teriorly where it is rounded and of moderate height, it is marked by a 

 median furrow to correspond with the median plication of the sinus, and 

 usually with two plications on each side ; lateral slopes marked by plica- 

 tions similar to those of the opposite valve in form and number. 



The entire surface of both valves is marked by fine, cencentric sublam- 

 ellose markings which are more conspicuous anteriorly, and sometimes by 

 occasional stronger lines of growth ; also by exceedingly minute longitud- 

 inal striae. 



Remarks. This species was originally described by Hall as a variety of 

 S. keokuk, the specimens illustrated by him being from "the mouth of 

 Lizard Creek, Webster County, Iowa." This locality is the site of the 

 city of Fort Dodge at the present time, and has afforded the type 

 specimens upon which the species has been established at this time. The 

 species differs from 8. keokuk in its smaller size, its more sharply defined 

 and more angular mesial sinus towards the beak, and especially in the 

 narrower and less gibbous umbonal region of the pedicle valve. The 

 species is perhaps most closely allied to the more transverse variety of 

 8. increbescens, from the Chester formations, but among these Chester 

 specimens there is never so great variation exhibited in the length of the 

 hinge-line, all of them being similar to the more transversely extended 

 examples of 8. pellaensis. 



Horizon. Pella beds of the Ste. Gene vie ve limestone. 



