396 



NORTH AMERICAN INDEX FOSSILS. 



72. N. corbuliformis Hall. (Fig. 503,/.) Devonic. 

 Distinguished by its small size and by its nearly equilateral 



triangular form. 



Hamilton : New York, Pennsylvania. 



73. N. houghtoni Stevens. (Fig. 503, a-b.) 



Upper Devonic and Mississippi. 

 Resembles N. randalli but is proportionally shorter and the 

 very fine concentric lines are uninterrupted by varices. 

 Chemung : Iowa. Waverly : Ohio, Michigan. 



74. N. ventricosa Hall. (Fig. 504.) Carbonic. 

 Very convex. Concentric striae fine and regular. 



Coal Measures : West Virginia to Colorado and Oklahoma. 



75. N. beyrichi von Schauroth. (Fig. 505.) 



Carbonic. 



Fig. 504. Nucula ventricosa. 

 (Kan. Pal., VI.) 



Fig. 505. Nucula beyrichi. 

 (Kan. Pal., VI.) 



Very small. Hinge line nearly rectangular at beaks. 



Coal Measures of Illinois, Kansas, Nebraska, Iowa. 

 76. N. cancellata Meek and Hayden. Cretacic. 



Subtrigonal, rather gibbous. Hinge line sloping abruptly an- 

 teriorly and posteriorly from the beak. Beaks slightly anterior 

 to the middle, and incurved nearly to touch each other. Surface 

 marked with radiating striae, crossed by fine concentric lines, so as 

 to form a cancellate sculpture. Length about 1 inch ; height 

 about I inch. 



Pierre : Montana, North Dakota. Fox Hills : South Dakota, 

 Nebraska, Colorado. Montana of Alberta. Also in Mexico. 



Fig. 506. A r ucula percrassa , internal molds. (Pal. N. J., I.) 



yy. N. percrassa Conrad. (Fig. 506.) Cretacic. 



Large. Surface covered with irregular concentric lines and fine 

 radiating costae most strongly developed anteriorly. Well pre- 

 served internal casts are strongly crenate on the margin. 



