230 PALEOZOIC PALEONTOLOGY. 
less conspicuous intermediate ones. The cardinal area is unsym- 
metrical and variable because of the attachment of the shell, but it is. 
always rather low; its margin is sometimes sharply defined, 
but is often very poorly defined. The brachial valve is more 
or less regularly convex, the greatest convexity usually being 
posterior to the middle; it is never so strongly marked by concentric 
wrinkles as the opposite valve. Both valves are marked by fine, 
more or less unequal, radiating ribs, of which three or four occupy 
a space of one millimeter at the margin of the shell. The larger ribs 
extend the entire length of the shell, while the smaller ones are inter- 
calated at various distances from the beak. 
The dimensions of a rather large pedicle valve are: length, 20.5 
mm.; width, 25.5 mm.; height of area at centre, 3 mm.; maximum 
depth of valve, 4.5 mm. A brachial valve having the same length and 
breadth has a convexity of 9 mm. 
Remarks.—This species differs greatly from any other Silurian 
species of the genus, having a much closer resemblance to younger 
forms such as O. chemungensis of the Devonian and some Carbonifer- 
ous species. It differs from any of these younger forms, however, in 
its much larger facet of attachment upon the umbo of the pedicle valve. 
CHONETES JERSEYENSIS Weller. 
Plate XX., Figs. 11-16. 
1900. Chonetes jerseyensis Weller, Ann. Rep. Geol. N. J. for 1899, 
De: 
Description.—Shell concavo-convex or nearly plano-convex, length 
about two-thirds the breadth, hinge-line usually a little shorter than 
the greatest breadth, lateral and anterior margins regularly rounded. 
Pedicle valve depressed-convex, the greatest convexity near the beak ; 
beak small, not prominent; cardinal area low, with as many as sever 
slightly-oblique marginal spines on each side of the beak upon the 
larger specimens. Brachial valve slightly concave or nearly flat. Both 
valyes marked by rather coarse, radiating ribs, which increase by im- 
plantation and bifurcation, three or four of them occupying a space 
of 2 mm. at the front margin. On the younger shells the ribs are 
usually finer and more angular and the lateral ones often have a 
