LAMELLIBRANCHIATA. 473 



PHTHONIA, Hall. 1870. 



r Types P. sectifrmis and P. nodimstata. ] 



Phthonia cylindrica. 



PLATE LXXVIII, FIGS. 1-4. 

 Phthonia cyliiidrica, 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. 



