400 PALJEONTOLOGY OF NEW YORK. 



Surface marked by fine, somewhat regular concentric striae, with obscure 

 indications of radiating striae upon the middle of the shell. The post- 

 cardinal slope is sometimes marked with fine undulating and bifurcating 

 vascular lines, which become obsolete below the umbonal ridge. 



Ligamental area narrow. Hinge marked by two short triangular cardinal 

 teeth in the right valve, and by one or two extremely slender lateral teeth. 

 Anterior muscular impression strongly marked ; posterior scar shallow. Pal- 

 lial line simple, distinctly marked by closely arranged radiating lines extend- 

 ing from above and terminating at the line. 



Five specimens measure respectively 29, 31, 32, 37 and 43 mm. in length, 

 and 13, 13, 15, 15 and 19 mm. in height. 



This species resembles S. cuneatus, but the umbonal ridge is not so strongly 

 marked, the cincture is a much less conspicuous feature, and the posterior end 

 is more pointed. 



Formation and localities. In the Chemung group at Philipsburgh and Rock- 

 ville, Alleghany county ; near Olean and Randolph, Cattaraugus county ; near 

 Ellington and north of Panama, Chautauqua county, N. Y. ; and at Warren and 

 Alleghany springs, Warren county, Pa. 



Sphenotus (?) ARCUATUS, n. sp. 



I'LATE LXVI, FIG. 18. 



In part SanffuinolUes rigidus (White anil Wiiitkikld), Hai.i.. Pal. N. Y., vol. v, pt. 1. Plates and Expla- 

 nations: PI. 66, figp. l-lt>. 1883. 



Shell large, elongate, sub-trapezoidal, arcuate ; length more than twice the 

 height ; biisal margin gently arcuate. Posterior extremity obliquely trun- 

 cate. Cardinal line long, straight, more than half the length of the shell, 

 parallel to the basal margin. Anterior end short, abruptly rounded below. 



Valves becoming gibbous in the middle and umbonal region. 



Beaks sub-anterior, appressed, incurved, rising but little above the hinge- 

 line. Umbonal slope obtusely angular, arcuate, declining gently to the 

 middle of the shell and then more abruptly to the post-inferior extremity. 



