43S THE PALEONTOLOGY OF MINNESOTA. 



LOrtliis (Plectorthis) vliitfleldi. 



has the fine cross-striation. The outward plications of the valve are strongly marked 

 on the cast for about two and a half lines from the margin, and some of them run 

 faintly even to the edge of the muscular scar. 



" The entering [dorsal] valve is much less convex, but cannot be said to be flat, 

 though it has a faint flattening along the center, which widens to the front margin 

 where it is changed, in the large specimen, to a slight concavity and produces a 

 straightening and also a very slight flexure of the margin. In front of the cardinal 

 angles also, on either side, is a flat, depressed area; cardinal angle parallel with the 

 posterior margins of the valve, and a little more than one-half the hight of that of 

 the receiving valve; beak indistinct; foramen triangular and about as wide as high, 

 with a small, central, smooth tooth [cardinal process], which does not rise above the 

 plain of the area and only becomes visible on being cleaned and excavated. A cast 

 of the interior of this valve shows marked internal characters. While the impres- 

 sions of the individual divaricator and adductor muscles of the same side are not 

 separable with certainty, owing to the faintness of the lines between them, the 

 pairs of each are divided, on the cast, by a deep, sharp furrow that extends from the 

 beak where it divides the divaricately striated cardinal process into two equal lobes, 

 toward the front between the depressions of the hinge teeth, to a point somewhat 

 more than one-third the diameter from the beak, where it dies away or runs into a 

 broad, abrupt, medial depression which produces the flatness in the valve extending 

 to the front margin. The external costse are deeply impressed on the cast about the 

 margin, some of the lines running faintly within the vascular area. The exterior 

 of this valve is also marked by concentric fine striations, especially between the 

 costae." 



The vascular trunks are often conspicuous in the ventral valve, having their 

 origin at the antero-lateral elevated margin of the muscular area; diverging slightly, 

 they proceed but a short distance forward. Posterior to these and on each side of 

 the muscular area are the faintly marked genital spaces. 



This species is closely related to 0. kankakensis McChesney,* but is always pro- 

 portionately less elongated along the hinge-line, and therefore squarer in outline. 

 The plications are also more numerous, there being from sixty to seventy along the 

 margin of one valve in that species, while in 0. whitfieldi there are usually not more 

 than forty. 



The specimen figured by Prof. Whitfield (op. cit.) as 0. pectinella (Emmons) Hall, 

 and found in the Cincinnati group at Delafield, Wisconsin, undoubtedly belongs to 

 this species. It does not occur above the lower portion of the Galena, while 

 0. whitfieldi is unknown below the Hudson Eiver group, and always has a greater 



*New Pal. Foss., p. 77. 1861; also Trans. Chicago Acad. Sol., vol. 1, p. 29. pi. ix. flg. 3, 1868. 



