43S THE PALEONTOLOGY OF MINNESOTA. 
[Orthls (Plectort)iis) whitfieldi. 
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 
costse." 
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. ivhiifieldi is unknown below the Hudson River group, and always has a greater 
*New Pal. Foss., p. 77, 1861; also Trans. Chicago Acad. Sci., vol. 1, p. 29, pi. ix. flg. 3. IStiS, 
