LAMELLIBRANCHIATA. 395 



Sphenotus arc^formis. 



PLATE LXV, FIGS. 7-11; AND PLATE LXVI, PIG. .13? 



SanguinoUtes arcceformis. Hall. Prelim. Notice Lamellibranchiata, 2, p. 40. 1870. 



" " Pal. N. Y., vol. V, pt. 1. Plates and Explanations : PI. 65, figs. 7-U. 1883. 



" Molus, Hall (in error). Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 66, fig. 43. 



1883. 



Shell of medium size, trapezoidal ; length usually more than twice the height ; 

 basal margin gently curved, sometimes nearly straight in the middle. Pos- 

 terior extremity obliquely truncate. Cardinal line straight, slightly declin- 

 ing posteriorly. Anterior end short, sloping rapidly from the beak and 

 narrowly rounded below. 



Valves becoming gibbous in the middle and umbonal region. 



Beaks at about the anterior fourth, small and closely appressed. Umbonal 

 slope distinctly angular, extending to the post-inferior extremity. Post- 

 cardinal slope concave, and marked by a scarcely defined fold which is nearer 

 to the umbonal ridge than to the cardinal margin. Most of the specimens 

 show a flattening or undefined depression extending from the beaks to the 

 basal margin. 



Surfsvce marked by extremely fine concentric striae, which sometimes 

 become fasciculate in the anterior portion of the shell, and are undulating 

 on the post-cardinal slope ; also by fine radiating striae, which are rarely 

 well-preserved, and are mainly confined to the posterior half of the shell. 



Five specimens measure respectively 23, 28, 32, 26 and 26 mm. in length, 

 and 11, 13, 14, 14 and 11.5 mm. in height. 



This species diifers from S. cuneatus in its smaller size, narrower posterior 

 extremity, more convex basal margin, less conspicuous post-cardinal ridge, and 

 in the surface characters. 



Formations and localities. In the shales of the Hamilton group, near Delphi, 

 at Pratt's falls and on the shore of Skaneateles lake, Onondaga county, N. Y. ; 

 and doubtfully in the Chemung group near Smethport, Pa. 



