LAMELLIBRANCHIA TA. 
477 
the base as a broad, undefined depression. Umbonal slope obtusely angular, 
extending to the post-inferior extremity. Post-cardinal slope wide, marked 
along the middle of its length by a distant fold. 
Surface marked by fine concentric striae which, on the posterior part of the 
shell, are somewhat fascicled. Surface also marked by fine, equal radiat¬ 
ing striae, giving the surface a somewhat cancellated appearance. The radii 
are obscure on the post-cardinal slope and are principally developed in the 
umbonal depression. 
The specimen described has a length of 39 mm. and a height, at the pos¬ 
terior extremity, of 18 mm. 
This species somewhat resembles P. sectifrons , but is broader and more 
abruptly truncate at the posterior end, and the umbonal ridge is more distinctly 
defined. 
Formation and locality. In the Chemung group, south-east of Panama, N. Y. 
Phthonia nitida, n. sp. 
PLATE XCIV, FIGS. 5, 6. 
Shell below the medium size, narrow-elliptical; length more than twice the 
greatest height; basal margin nearly straight, gently curving upward at the 
anterior and more rapidly at the posterior. Posterior extremity rounded, a 
little oblique above. Cardinal line nearly straight, or slightly arcuate. 
Anterior end short, narrowly rounded. 
Yalves moderately convex below and posteriorly, becoming gibbous in the 
middle and umbonal region. 
Beaks sub-anterior, small, incurved and appressed. Umbo flattened, and 
this feature is continued, as a broad undefined depression, to the basal margin. 
Umbonal slope convex, not defined, merging into the general convexity 
• before reaching the post-inferior extremity. 
Surface marked by fine, filiform concentric strise, which sometimes become 
fasciculate on the posterior and basal portions of the shell; and by fine radi¬ 
ating strise extending from the beaks over the entire surface, being stronger 
61 
