LAMELLIBRANCHIA TA. 
145 
Hinge-line straight, length about one-seventh less than the greatest length 
of the valve. 
Beaks at about the anterior fourth, prominent and incurved over the hinge¬ 
line in the left valve; depressed in the right valve and not rising above the 
cardinal line. Umbonal angle nearly rectangular. 
Anterior end large, limited by a broad, slightly oblique, undefined byssal 
depression ; extremity acute and nasute. Wing elongate triangular, joining 
the body at the posterior end; the dividing furrow distinct in the left valve, 
more conspicuous in the right; the fold is not defined in the left valve, and is 
strongly marked in the right; margin nearly vertically truncate; extremity 
not perceptibly produced. 
Test thin, marked by fine radii, which are conspicuous upon the body and 
wing, and obsolete on the anterior end and byssal depression. The surface 
is also marked by fine concentric striae, which are crowded into fascicles, 
giving an apparent undulation to the surface from the umbo to the lower 
extremity, especially in the right valve. At the post-basal margin the 
striae are recurved at a little less than a right angle in the left valve, and 
more acutely in the right valve, and they are strongly lamellose on the 
anterior end. 
Interior unknown. 
The specimen described has a length of 56 nun., height 30 mm., and 
hinge-line about 48 mm. 
This species resembles P. lata, and also some forms of P. Sao, but it is much 
more elongate, the valves more depressed, the wing narrower and longer. 
Formation and locality. In the Upper Chemung group at Warren, Pa. 
Ptychopteria lata, n. sp. 
PLATE LXXXV, FIGS. 23-25. 
Shell large, rhomboidal; body sub-elliptical, very oblique, making an angle 
with the hinge of nearly 40°; height equal to about two-thirds of the length ; 
ante-byssal margin oblique, curving into the shallow sinus, from whence the 
19 
