ORDOVICIAN FAUNAS. 151 
mens, or strongly elevated in flatter ones. Delthyrium as wide or 
wider than long, filled with a conspicuous, convex deltidium, which is 
broadly excavated anteriorly and occupied by the chilidium. Brachial 
valve flattened or slightly concave in the umbonal region, more or 
less strongly convex laterally and anteriorly, often with a shallow, 
narrow sinus extending from the beak to the middle of the shell, and 
with a broadly-rounded median fold near the front margin; cardinal 
area narrow, vertical, centrally occupied by a broad and short chili- 
dium.. Surface of both valves marked by numerous, fine, subequal, 
crowded, rounded or subangular cost, which increase by intercala- 
tion, every second, third or fourth one being more prominent. The 
whole surface is also crossed by numerous, crowded, delicate, raised, 
concentric lines and by a few stronger marks of growth. In some 
specimens oblique wrinkles are present along the cardinal margin on 
each side of the beaks. 
The dimensions of a medium-sized brachial valve are: length, 17 
mm.; width, 23 mm.; convexity, 8 mm. 
Remarks: —This species occurs quite commonly in the Trenton 
fauna. In its more or less imperfect condition of preservation it may 
sometimes be confused with Rafinesquina alternata, although its 
brachial valve, and not its pedicle valve, is convex. On the convex 
valve of 8. incurvata, however, there is always a slight flattening or 
shallow concavity near the beak, while in R. alternata the same por- 
tion of the shell is slightly convex. The delicate concentric surface 
markings of this species are characteristic, but these are usually de- 
stroyed upon the New Jersey specimens, because of their manner of 
preservation. 
ORTHIS TRICENARIA Conrad. 
Plate IX., Figs. 18-21. 
1892. Orthis tricenaria Hall and Clarke, Pal. N. Y., vol. VIIL., pt. 
plo tigs) Oia 
Description.—Shell plano-convex, longitudinally semi-elliptical in 
outline; hinge-line equal to the greatest width of the shell, rarely 
shorter. Cardinal area well developed on each valve. Surface marked 
by thirty to thirty-six usually nearly equal, simple, subangular, radi- 
ating cost, which are crossed by exceedingly delicate, concentric lines 
