LAMELLIBRANCHIATA. 453 



Valves moderately convex below, and gibbous in the middle and above. 



Beaks in advance of the anterior third, prominent, rising considerably 

 above the hinge-line, and incurved. Umbonal slope distinctly angular, 

 extending to the post-inferior extremity. 



Surface marked by fine concentric striae, which are scarcely preserved in 

 the cast. 



Two specimens measure respectively 18 and 20 mm. in length, and 16 and 

 18 mm. in height. 



This species is distinguished by its rhomboid, erect form, its nearly equal 

 length and height, and the divergence of the umbonal slope from the cardinal 

 line, in which respect it resembles S. quadrangularis, but is a nmch smaller 

 form. 



Figure 20 is of one of the original specimens from the sandstone of the age 

 of the Panama conglomerate, while the others are from lower beds. Larger 

 collections may show the necessity of separating these two forms, which are 

 from very distinct horizons. 



Formation and localities. In the sandstone of the Panama conglomerate, 

 Chautauqua county, and from the middle portion of the Chemung group, near 

 Hobbieville, Alleghany county, N. Y. 



ScHizoDcs Chemungensis. 



PLATE LXXV, FIGS. 37-40, 45, 41 ?. 



Nuculitis ChemungeniMS, Conrad. Jcjui-. Acad. Nat. Sci., Phila., vol. viii, p. 247, pi. 13, tig. 13. 1842. 

 SehUodun Chemungeiutis (Conrad), Hall. Pi-elim. Notice Lamellibranchiata, 2, p. 96. 1870. 

 Cytherodnn (Sihizodus) ChemungeiiKis (Conrad), Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: 



PI. 75, figs. 37-40. 1883. 



Shell large, rhomboid-ovate; length one-fourth greater than the height. 

 Anterior margin broadly rounded, curving into the basal margin, which is 

 sometimes nearly straight posteriorly. Post-inferior extremity angular. 

 Posterior margin obliquely truncate. Cardinal line straight, less than half 

 the length of the shell. 



Valves depressed-convex below, becoming gibbous in the middle and 

 58 



