WAVERLY GROUP SPECIES. 



compressed, and not projecting above the hinge margin or showing the 

 slightest obliquity; superior lateral regions compressed and separated 

 from the more convex central portion by shallow undefined impressions 

 that diverge from each side of the beaks at an angle of about 90°, the 

 one on the posterior side being usuallj^ longer than the other. Surface 

 appearing nearl}' smooth, but showing under a magnifier very fine, reg- 

 ular, crowded, and obscure concentric stria3, with a few finely marked, 

 irregular furrows of growth, and sometimes the slightest possible traces 

 of radiating strisB, that are generally entirely obsolete. 

 Height, 1.04 inches; breadth, 1.08 inches. 



The foregoing description was mainly drawn up from the specimen 

 represented by our figure 4a. This I believe to be a left valve. Other 

 specimens in the collection, however, present the form shown by our fig- 

 ure 46. These are believed to be the right valve of the same species, and 

 will be seen to have the hinge line straight, and the superior lateral 

 margins more protuberant in outline, so as to give a different form to the 

 whole valve. I have observed precisely the same difierences between 

 what we have every reason to believe to be the opposite valves of a 

 scarcely distinguishable species found in the Coal Measures, as may be 

 seen by figures given on plate 9 of Palaeontology of Eastern Nebraska, 

 published in Hayden's Nebraska Report of 1872, and on plate 26 of the 

 fifth volume of the Illinois Geological Reports. 



I am in considerable doubt in regard to the specific name that ought 

 to be retained for this shell. It perhaps agrees most nearly with Prof. 

 WinchelF description of his Pernopeden Shumardianus. But after seeing 

 how these shells vary in the slight details of form and their obscure sur- 

 face markings, it seems to me quite as probable that it may belong to his 

 P. Itmatus, or rather that the latter and P. Shumardianus may both belong 

 to one species, also including our shell. Prof. Winchell did not see the 

 hinge in either of these forms, but merely referred them to his genus 

 Pernopeden from their general external resemblance to the type of that 

 group. I feel well assured, however, that at least the specimens here 

 under consideration have not the crenate hinge characterizing Pernopec- 

 ten, and can not be properly referred to that group. 



Again, it certainly bears very close relations to Avicula Cooperensi?, 

 Shumard, from rocks of the same age in Missouri. In making compari- 

 sons with the latter, however, it should be kept in mind that the figure 

 of Shumard's species, given in the Missouri report, was drawn from a 

 very aberrant specimen, which also had its ears partly hidden by the 

 rock, so as to cause a misapprehension in regard to their form. The 



