474 PALEONTOLOGY OF NEW YORK. 



PhTHONIA NODICOSTATA. 



PLATE LXXVUl, FIGS. 5-9. 



Phthunia nodicostata. Hall. Prelim. Notice Laiuellibraiichiata, 2, p. 71. 1870. 



" " Pill. N. Y., vol. V, pt. 1. Plates and E.\planatioii8 : PI. 78, fij^s. 5-9. 1883. 



Shell of medium size or larger, rhomboid-obovate ; length about twice the great- 

 est height as measured at the posterior end. Basal margin nearly straight 

 along the middle, curving gently upward anteriorly and more abruptly pos- 

 teriori}'. Posterior extremity obliquely sub-truncate above and rounded 

 below. Cardinal line straight, about half the length of the shell, oblique, 

 ascending posteriorly. Anterior end short, somewhat narrowly rounded. 



Valves moderately convex below, becoming somewhat gibbous in the mid- 

 dle and umbonal region. 



Beaks sub-anterior, small, low, appressed and scarcely rising above the 

 hinge-line. Shell somewhat flattened from the beaks backward to the base. 

 Umbonal slope sub-angular above, gradually merging into the general con- 

 vexity of the shell before reaching the post-inferior extremity. 



Surface marked by fine concentric,' sub-lamellose striae ; also by strong 

 radii, extending from the beak over every portion of the shell, and which 

 are more distant from each other on the cardinal slope and anterior end. 

 These radii, at the crossing of the concentric strioe, are marked by transverse 

 nodose or pustulose elevations. 



Four specimens measure respectively 23, 31.5, 34 and 35 mm. in length, 

 and greatest height at the posterior end, of 12, 16, 16 and 17 mm., while the 

 height at the beaks is 10, 14, 14 and 14 mm. 



This species is distinguished by its great posterior elevation and the strong 

 radii marking the surface. 



Formation and localities. In the shales of the Hamilton group, on the shores 

 of Cayuga and Canandaigua lakes ; and near Oneonta, N. Y. 



