86 
BULLETIN OF THE LABORATORIES 
middle, on either side of the mesial sinus. Beak slightly elevated, 
scarcely incurved ; foramen broad-triangular, width equal to twice the 
height. Cardinal process small, laterally compressed, not filling the 
foramen, in a line with the base of the cardinal area ; cardinal area 
moderate, one third that of the ventral valve, equaling the hinge line, 
directed backwards, arched. Postero-lateral regions of the valve 
moderately compressed. 
Ventral valve more convex, its greatest convexity about two-fifths 
the distance from the beak, sloping regularly to the lateral and anterior 
margins. Beak prominent, incurved, having two or three times the 
elevation of the dorsal valve; cardinal area corresponding, directed 
obliquely backwards, arched ; foramen triangular. 
Surface marked by 40 to 50 radiating striae, which increase by fre- 
quent intercalations, and are crossed by distinct concentric striae, giv- 
ing the surface a beautifully ornate appearance, which will serve to 
readily distinguish it from the other species of Orthis found here. 
(Figs. 15 a, b, c, d.) 
A form of this species occurs having the general shape and char- 
acteristics of the typical specimens, but the plications are more angular, 
sometimes almost acutely ridged, and crossed by concentric striae, 
more closely arranged, and also much less distinct, not producing the 
ornate appearance characteristic of the typical form. For this reason 
I was once inclined to separate them under a different name, acuto- 
pUcata^ but at present I am of the opinion that they are not sufficiently 
distinct to be placed even under a varietal name. (Fig. 16 h.) 
Length of a specimen, 17 mm.; breadth, 21 mm.; convexity, 9 mm.; 
varying from this to specimens with a convexity of only 6 or 7 mm. 
This species seems to be a form intermediate between Orthis in- 
sciilpta and O. bella-rugosa of the Lower Silurian strata, and O. Nisis of 
the Niagara group of Kentucky. It differs from the last in the much 
smaller elevation of the ventral beak ; from the second, in the coarser 
and broader appearance of its radiating striae, the absence of an in- 
curved anterior margin, and by the greater curve of its ventral beak ; 
from the first it is chiefly distinguished by greater size and more 
numerous striae. 
Locality and position. Soldiers’ Home Quarries, Clinton Group ; 
the typical forms, in the upper part of the group ; the variety with 
more acute plications, in the lower portions of the same. 
