396 PALEONTOLOGY OF NEW YORK. 



SpHENOTUS C0NEATUS. 



PLATK LXV, FIOS. li-17. 



Plerinea eamata, Cohrad. Geol. Sui-\'. N. Y., Ann. Rep., p. 1 IG. 1838. 



/Saiiffuhuilite* luueatux (Conrad), Hall. Pi-elini. Notice Laniellibranchiata, 2, p. 39. 1870. 



" " " " Pnl. N. Y., vol. V, pt. I. Plates and Kxplanation.s : PI. G."), tig». 



12-17. 1883. 



Shell large, trapezoidal ; length more than twice the height ; basal margin con- 

 stricted anterior to the middle and gently curving to both extremities. Pos- 

 terior extremity obliquely sub-truncate, sinuate. Cardinal line straight. 

 Anterior end short, rapidly declining from the beak and narrowly rounded 

 below. 



Valves depressed-convex posteriorly, moderately convex below the middle 

 and becoming somewhat gibbous in the umbonal region. 



Beaks sub-anterior, small, flattened and closely appressed. Umbonal slope 

 marked by an obtusely angular fold or ridge, extending to the post-inferior 

 extremity. Post-cardinal slope concave, marked along its centre by a low 

 fold, which is a little nearer the cardinal line than to the umbonal ridge, 

 extending to the posterior extremity. Cincture oblique, extending from the 

 beak to the base of the shell, producing a depression of the surface and a 

 sinuosity in the margin. 



Surface marked by fine concentric striae, which become fasciculate and 

 often form low undulations anterior to the umbonal ridge ; also by fine radi- 

 ating strisB on the body of the shell, which have not been observed on the 

 post-cardinal slope. 



Four specimens measure respectively 33, 42, 42 and 48 mm. in length, and 

 14, 17, 18 and 19 mm. in height. 



This species differs from S. arceformis in its proportionally greater length, 

 with a greater height at the posterior end ; in its sinuate posterior margin, 

 constricted basal margin and more regular concentric striae. 



In certain conditions of preservation, the radiating striae are obscure or 

 ob.solete, and sometimes worn by weathering, and are therefore not to be relied 

 upon as characterizing the species in all cases. 



