CONTRIBUTIONS TO PALEONTOLOGY. 25 



This species is a well-marked and very distinct form, occurring in the 

 thin arenaceous layers at Cuyahoga falls, and in the green shale at Akron, 

 Ohio, in strata referred to the upper part of the Chemung group of New- 

 York, or Waverly sandstone of Ohio. 



GENUS DISCINA ( Lamarck), 



*' DISCINA HUMILIS (n.s.)." 



Shell of medium size, circular or subcircular, very depressed- 

 convex on the dorsal side ; apex subcentral. Yentral valve flat, 

 with apex subcentral ; foramen apparently submarginal. 



Surface, from the apex halfway to the margin, marked by fine 

 concentric striae, and outside of this, by a few comparatively 

 distant sharp elevated striae, with the intermediate spaces 

 scarcely perceptibly striate. 



Two specimens only of this species have heen recognized : the larger of 

 these has a diameter of more than an inch ; and the smaller one, ahout 

 three-fourths of an inch. They are more nearly circular than any other 

 species in the Marcellus shale and Hamilton group, except the D. minuta. 

 The D. lodensis is sometimes circular ; but its prevailing form is broad- 

 oval, and it is always closely and finely striated, and, in this feature, very 

 distinct from the present species. 



Geological formation and locality. In the Marcellus slate, near Bridg- 

 water ; and in the shales of the Hamilton group, on Canandaigua lake. 



>' DISCINA RANDALLI (n.s.)." 



Ventral valve circular or nearly circular, gently concave within 

 the margin : foramen large, marked by a broadly oval depres- 

 sion on the exterior surface, which reaches half the distance 

 from the apex to the margin. 



Surface marked by strong rounded concentric ridges with sharp 

 depressions between, and sometimes with finer concentric strias 

 upon the coarser ones ; all of them crossed, on the posterior 

 margin of the shell, by fine radiating striae or vascular im- 

 pressions, which enter into the substance of the shell. On the 

 anterior half of the shell the concentric strise become partially 

 obsolete, and, from a constricted ridge which externally marks 

 the place of a median septum in the muscular impression, di- 

 verge strong rounded radiating ridges, separated by narrow 

 abrupt depressions which extend nearly or quite to the margin 

 of the valve. 



[Senate, No. 115. J 4 



