152 PALEOZOIC PALEONTOLOGY. : 
of growth. Pedicle valve strongly convex, subangular along the 
median line, with the greatest elevation on the umbo. Cardinal area 
very high, more or less concave, striated longitudinally and trans- 
versely, divided by a very narrow delthyrium, whose apical third is 
occupied by a flat, concave or convex deltidium. Brachial valve nearly 
flat, slightly elevated at the beak, from which point the surface slopes 
gradually into a broad, scarcely-perceptible, rarely well-defined, me- 
dian sinus. Cardinal area nearly one-third as wide as that of the 
pedicle valve, flat, divided by a triangular delthyrium, which is as 
broad as long and more or less covered by a convex chilidium, the 
anterior margin of which is concave. 
The dimensions of a rather large specimen are: length, 19 mm.; 
width, 20 mm.; thickness, 10.5 mm. 
Remarks.—The presence of this species always indicates a low Tren- 
ton horizon, it being- entirely absent from the higher beds.- The New 
Jersey specimens are always poorly preserved, being more or less ex- 
fohated, the two valves being rarely found in articulation. 
PLECTORTHIS PLICATELLA (Hall). 
Plate IX., Figs. 22-24. 
1847. Orthis plicatella Hall, Pal. N. Y., vol. L., p. 122, pl. 32, igag: 
1892. Plectorthis plicatella Hall and Clarke, Pal. N. Y., vol. VIII, 
pt. 1. p.194, pl. 5, figs: 18=20. 
1895. Orthis (Plectorthis) plicatella Winchell and Schuchert, Pal. 
Minn., pt. I., p. 436, pl. 33, figs. 5-7. 
Description.—Shell lenticular, transversely subelliptical in outline, 
each valve marked by from twenty-two to thirty-two strong, simple, 
radiating plications. In some specimens a few much-finer plications 
are intercalated between the primary ones near the front margin. 
Hinge-line shorter than the greatest width of the shell; cardinal ex- 
tremities angular or somewhat rounded. Pedicle valve regularly con- 
vex, the greatest elevation being at or a little back of the middle. 
In front of the middle point the mesial portion is usually slightly 
flattened, but never enough to make a mesial sinus. The cardinal 
area is sharply defined, broadly triangular and moderately concave. 
The brachial valve is usually a little less convex than the pedicle, its 
greatest elevation being near the middle. No mesial fold is developed. 
