LAMELLIBRANCHIATA. 401 



Post-cardinal slope broad. Cincture extending from the beak obliquely to 

 the base, becoming broatl below the middle of the shell. 



Surface marked by fine concentric striaj, the impressions of which are 

 preserved upon the cast. 



Anterior muscular impression deeply marked, limited posteriorly by a 

 ridge, situated just below the beaks. Pallial line continued parallel to the 

 basal margin, recurving over the umbonal ridge and terminating in a large 

 muscular scar on the post-cardinal slope. 



The specimen described has a length of 50 mm. and a height of 2?> mm. 



•This species .somewhat resembles S. subtoHuosus, but the umbonal ridge is 

 more arcuate, the cardinal slope is wider and without distinct plication. 



Formation and locality. In a conglomerate of the Chemung group, east of 

 Panama, N. Y. 



Sphenotus clavulus. 



PLATE LXVI, FIGS. 2, 20-26. 



SaugaiHolite* f claenlitu, Hall. Pi-elim. Notice Lamellibranchiata, 2, p. 45. 1870. 



" " " Pal. N.Y.,vol. V, pt. 1. Plates and Explanations : PI. C6, figs. 20-26. 1883. 



Shell of medium or large size, elongate, sub-trapezoidal ; length more than 

 two and a half times the height; basal margin nearly straight or slightly 

 arcuate in the middle, curving to the anterior and posterior extremities. 

 Posterior extremity narrowly rounded below and oblique above. Cardinal 

 line straight, .sli^itly declining posteriorly, nearly parallel to the basal mar- 

 gin and extending more than half the length of the shell. Anterior end 

 declining more or less abruptly from the beaks and narrowly rounded below. 



Valves moderately convex below, gibbous in the middle, and on the umbo- 

 nal slope to beyond the middle of its length. 



Beaks sub-anterior, small, closely appressed, incurved, and rising very 

 little above the hinge. Umbonal slope sub-angular from the beak to the 

 middle of its length, becoming rounded and less defined below and reaching 

 the post-inferior extremity. Post-cardinal slope moderately convex, with 



