Weller — KinderJwok Faunal Studies. 25 



Surface of the shell ornamented with very fine, regular, 

 crowded, concentric striae which are often scarcely recogniz- 

 able without the aid of a lens. Aside from the concentric 

 striae there are sometimes present ill-defined radiate costae 

 which on many specimens are wholly obsolete. 



Hemarks. In 1855 Shumard * described Avicula cooper- 

 ensis from the Chouteau Limestone of Cooper County, Mis- 

 souri, but his figure of the type specimen, according to Meek f 

 who examined it, along with many others from the same 

 locality, is misleading in its representation of tlie strong 

 radiating ribs and in the form of the ears. In regard to this 

 illustration Meek says: "the figure of Shumard's species, 

 given in the Missouri report, was drawn from a very aberrant 

 specimen, Avhich also had its ears partly hidden b}' the rock, 

 so as to cause a misapprehension in regard to their form. 

 The radiating costae seen on Shumard's figure, are also made 

 much too strong by the engraver, even for the individual 

 specimen there represented, while in a great majority of 

 specimens no traces of them whatever are to he see/i." 



In 1858 Hall t indentified a species from the " Chemung 

 Group" at Burlington, Iowa, with Shumard's ylv2ci<?a circulus^ 

 and illustrated it in his report. This illustration is very dif- 

 ferent from Shumard's original figure § of the species to which 

 it is referred, but agrees very well with his figure of A. 

 coopere7isis except for the radiating ribs which Meek says 

 should not be there. 



In 1862 White and Whitfield |1 described without illustra- 

 tion, Aviciilopecten limaformis^ also from the " Chemung 

 Group " at Burlington, Iowa, 



In 1865 Winchell^ described without illustrations the genus 

 Pernopecten, with Aviculopecten limaformis W. & W^. as the 

 type species. The genus was characterized by the presence 

 along the hinge-line of a central, triangular cartilage pit and 



* I and II Rep. Geol. Surv, Mo. 206. PI. C.f. 15. 



t Rep. Geol. Surv. Ohio 2^: L^93. 



X Geol. Iowa V : 522. pi. VII. f. 9. 



§ I and II Rep. Geol. Surv. Mo. pi. C. f. 14 a-b. 



ti Proc, Bost. Soc. Nat. Hist. 8 : 295. 



1 Proc. Acad. Nat. Sci. Phil. 1866 : 126-126. 



