LAMELLIBRANCHIATA. 475 



Phthonia sectifrons. 



PLATK LXXVIII, FIGS. 10-13. 



Cyprivardittn stctifnms, Conhad. Jour. Acail. Nat. Sci., Phila., vol. viii, p. 245, pi. 13, fig. 8. 1842. 

 Pktltmiia sectlfruiis (Conrad), Hall. Prelim. Notice Lamellibrauchiata, 2, p. 70. 1870. 



Pal. N. Y., vol. V, pt. 1. Plates and Explanations : PI. 78, 

 fi^. 10-13. 1883. 



Shell above the medium size, elongate, sub-elliptical, somewhat obovate ; 

 length more than twice the height measured at the posterior end ; basal 

 margin nearly straight in the middle, curving gradually at the anterior and 

 more abruptly at the posterior end. Posterior extremity rounded below, 

 oblique and sub-truncate above. Cardinal line straight, more than half the 

 length of the shell, slightly oblique, rising posteriorly. Anterior end short, 

 narrowly rounded. 



Valves depressed-convex along the basal and posterior portions, becoming 

 somewhat gibbous in the middle and above. 



Beaks sub-anterior, small, low and appressed. Umbonal slope obtusely 

 sub-angular in the upper part, usually becoming merged in the general con- 

 vexity before reaching the post-inferior extremity. Above this, and slightly 

 diverging, is another low ridge extending to the middle of the posterior end, 

 and limitiug the post-cardinal slope. 



Surface marked by fine concentric striae, and by numerous fine radii 

 extending from the beak to all parts of the shell. The radii on the umbonal 

 ridge and anterior end are stronger than those on the middle of the shell. 

 At the crossing of the radii and concentric striae, the test is raised into 

 minute elongate nodes, and'on the post-cardinal slope the concentric striae 

 and radii are nearly of equal strength, giving a cancellated and peculiar aspect 

 to the surface. 



Three specimens measure respectively 26, 39 and 40 mm. in length, and 

 13, 15.5 and 17 mm. in height at the posterior end. 



This species differs from P. nodicostata in its more elongate form, and much 

 finer radii. 



Formation and localities. In the shales of the Hamilton group, in Madison 

 county; and at Pratt's falls, in Onondaga county, N. Y. 



