Preliminary Notes on Cincinnatian Fossils 
223 
convex than the brachial, owing to the much greater height of 
the hinge area. Hinge teeth projecting but slightly beyond the 
cardinal line, supported by vertical plates uniting with the pos- 
terior part of the lateral border of the muscular area. Muscular 
area about 3.3 mm. in width and 4 mm. in length in a shell having 
a total length of ii millimeters. The muscular area rests upon 
a callosity thickening anteriorly to a height of about three-fourths 
of a millimeter above the general surface of the interior of the 
valve. The muscular scar is divided lengthwise by two low stria- 
tions into three divisions of which the middle one is slightly nar- 
rower, but projects a little farther anteriorly. In one case a narrow 
median striation is found along the central division of the muscular 
area. It is assumed that the lateral divisions correspond to the 
diductor muscular scars. How much of the middle division cor- 
responds to the adductor scars is unknown. 
Geological position. The types come from the Lower Eden, at 
Roger’s Gap, Ky. They occur from this locality northward as 
far as Sadieville. Southeastward they may be traced as far as 
Hutchison in southwestern Bourbon county, and Riverside, in the 
southern part of Clark county. Along the Ohio river they have 
been traced from Cincinnati to Higginsport. They appear to be 
confined to the lower part of the Lower Eden. 
The deep median groove in the initial stages of the brachial 
valve, and the well marked fold and sinus anteriorly on the two 
j valves, produce a form quite distinct from the more typical forms 
; referred to Plectorthis. For the group of shells typified by Plect- 
! orthis nicklesi and Plectorthis rogersensis, the subgeneric term 
I Eridorthis is proposed. 
Plectorthis (Eridorthis) rogersensis, nov. sp. 
I {Plate IV, Figs. 4 A, B.) 
A second form, evidently so closely related to Plectorthis nicklesi 
I as to be possibly only a variety, occurs at the same localities as 
I that species, and at the sarnie horizon. It appears to be more 
I common at Roger’s Gap than Plectorthis nicklesi^ and differs 
, chiefly in having distinctly fewer radiating plications, on the sinus, 
in the fold, and also laterally. The plications are broader, and 
the concentric striations are stronger and more distant. 
