156 PALEOZOIC PALEONTOLOGY. 
DALMANELLA SUBAEQUATA. (Con.). 
Plate X., Figs. 3-4. 
1843. Orthis subequata Conrad, Proc. Acad. Nat. Sci. Phil., vol. L., 
p. 333. 
1847. Orthis subequata Hall, Pal. N. Y., vol. L., p. 118, pl. 32, fig. 2. 
°1892. Dalmanella subequata Hall and Clarke, Pal. N. Y., vol. VIIL., 
pt. I., p. 207, pl. 5c, figs. 6-11. 
1895. Orthis (Dalmanella) subaquata Winchell and Schuchert, Pal. 
Minn., pt. I., p. 446, pl. 33, figs. 30-36. 
Description.—Shell subequally biconvex, usually wider than long; 
the hinge-line shorter than the greatest width of the shell, except 
sometimes in young individuals; cardinal extremities angular or 
rounded. Surface of each valve marked by numerous, fine, tubulose 
strie, which bifurcate about twice in passing from the beak to the 
anterior margin. Pedicle valve strongly and evenly convex, the great- 
est elevation posterior to the middle of the shell; near the beak and 
upon the umbo no mesial depression exists, but near the middle of 
the valve a broad, shallow and indistinct sinus begins and becomes 
deeper toward the anterior margin. The cardinal area is well de- 
fined, broadly triangular, elevated and only moderately concave; the 
delthyrium, with slightly-curved sides, is about twice as high as wide. 
Brachial valve more evenly, but a little less, convex than the pedicle, 
the greatest elevation near the middle. Near the beak the mesial 
portion of the shell is usually flattened or slightly depressed, but near 
the middle of the shell this flattening gradually changes into a low, 
broad, ill-defined elevation, corresponding with the sinus of the pedicle 
valve. The cardinal area is narrow and concave, with a delthyrium 
as broad or broader than high. 
‘The dimensions of an average specimen are: breadth, 16 mm., and 
length, 14 mm. 
Remarks.—Vhis species occurs in great numbers in the lower beds 
of the Jacksonburg section. It exhibits some variation in its size and 
propertions, one of the largest specimens observed being 25 mm. in 
breadth, 19 mm. in length, and with a thickness of both valves of 15 
mm. In immature specimens the ill-defined sinus of the pedicle valve 
dors not exist, and in many adult specimens the low, broad fold of 
the brachial valve is nearly or quite obsolete. 
