LAMELLIB RA NCHmA BA. 
473 
PHTHONIA, Hall. 1870. 
[ Types P. sectifrons and P. nodico.data. ] 
Phthonia cylindrica. 
PLATE LXXVIII, FIGS. 1-4. 
Phthonia cylindrica. Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 78, fig-s. 1-4. 1883. 
Shell of medium size, elongate-elliptical or sub-elliptical; length more than 
twice the height; basal margin straight or slightly concave, curving into the 
posterior and anterior margins without interruption. Posterior extremity 
regularly rounded, sometimes somewhat oblique on the upper posterior mar¬ 
gin. Cardinal line very gently arching or nearly straight, about half the 
length of the shell. Anterior end short, narrowed, and regularly rounded at 
the extremity. 
Valves moderately convex in the lower portion, scarcely gibbous in the 
middle and upper portion. 
Beaks sub-anterior, low, appressed, with sometimes a faint indication of a 
depression extending from the beak to the basal margin Umbonal slope 
not defined, rounded, arcuate, becoming obsolete before reaching the post¬ 
inferior extremity. 
Surface marked by fine, even concentric striae, and by extremely fine radi¬ 
ating striae, which are more conspicuous along the middle and posterior por¬ 
tions of the valve below the umbonal slope. (These striae are not sufficiently 
indicated in the figures.) 
Anterior muscular scar distinct. 
Four specimens measure respectively 26, 27, 29 and 30.5 mm. in length, 
and 11, 12, 12 and 13 mm. in height. 
This species differs from the others described in its more elongate-elliptical 
form, wider anterior end, and finer radiating striae, which are frequently obso¬ 
lescent and often obsolete. 
Formation and localities. In the shales of the Hamilton group, at Delphi ; at 
Pratt’s falls, Onondaga county; and on the shore of Cayuga lake, N. Y. 
