398 PALEONTOLOGY OF NEW YORK. 



strongly developed, and becoming obsolete toward the post-inferior extremity ; 

 the posterior margin is distinctly doubly truncate, with the lower truncation 

 vertical, and the cincture is more strongly marked. 



Formation and localities. In the shales of the Hamilton group, at Fultonham, 

 Schoharie county, and at York, Livingston county, N. Y. 



Sphenotus solenoides. 



PLATE LXV, FIGS. 21-29. 



Satiffuinolites solenoides. Hall. Prelim. Notice Lamellibranchiata, 2, j). 38. 1870. 



" " Pal. N. Y., vol. V, pt. 1. Plates and Explanations : PI. 65, figs. 21-29. 1883. 



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

 nearly straight, slightly arcuate in the centre, curving toward the anterior 

 end. Posterior extremity obliquely truncate. Cardinal line straight, about 

 half the length of the shell, parallel to the basal line. Anterior end long, 

 gradually declining from below the beaks and narrowly rounded at the 

 extremity. 



Valves moderately convex below and posteriorly, becoming somewhat gib- 

 bous in the middle and umbonal regions. 



Beaks situated about two-fifths of the length of the shell from the ante- 

 rior end, small, appressed and incurved, rising but little above the hinge- 

 line. Umbonal slope distinctly angular, extending to the post-inferior 

 extremity. Post-cardinal slope flat or slightly concave, with an obscure fold 

 along the middle, extending to the posterior extremity. Cincture indistinct, 

 appearing as a flattening or slight depression, extending from the beak to 

 the basal margin anterior to the middle. (This feature is frequently obso- 

 lete, except in well-preserved specimens.) 



Surface marked by fine concentric striae, which are fasciculate, producing 

 more or less distinct folds or undulations upon the anterior and middle por- 

 tions of the shell, and- bending abruptly upward on crossing the umbonal 

 ridge. The posterior half of the shell, especially above the umbonal ridge, 

 is further marked by fine, irregular, elevated vascular lines. In some speci- 



