210 



NORTH AMERICAN INDEX FOSSILS. 



65. D. reticulata Hall. (Fig. 247.) Siluric. 



Shell gibbous posteriorly and more attenuate in front. Pedicle 

 valve with small, acute and closely incurved beak which is flattened 

 on the back, forming a " depressed plate." 



Niagaran of Ohio, Indiana, Wisconsin. 



Fig. 248. Billingsella coloradoensis, dorsal and ven- 

 tral valves, natural size. (After Hall.) 



FlG. 247. Dictyonella 

 reticulata, dorsal and car- 

 dinal views, X 2 - (After 

 Hall. ) 





FlG. 249. lYisusia festinata. (After Walcott. ) 



XXVIII. Billingsella Hall and Clarke. 

 Small, subquadrate or semicircular; hinge line straight. Pedicle 

 valve the larger and convex; cardinal area moderately high, ver- 

 tical or but slightly incurved, with a strong, arching deltidium. 

 Brachial valve flat or concave ; cardinal area lower than in pedicle 

 valve but usually also possessing a convex deltidium. Dental 

 plates continued along the bottom of the valve enclosing a small 

 muscular area near the apex. Entire surface covered with radiat- 

 ing plications. Cambric-Siluric. 



66. B. coloradoensis (Shumard). (Fig. 248.) Upper Cambric. 



Differs externally from Protorthis billingsi in being less transverse 

 and in that the brachial valve lacks both sinus and cardinal area. 



From Texas to Missouri, Minnesota, Wisconsin, Montana, 

 Wyoming and Idaho. 



XXIX. NisusiA Walcott. 

 Differs from Billingsella in that the radiating ribs support irregu- 

 larly distributed spines on their crests ; and in the presence of a 

 mesial sinus on the pedicle valve and a faint mesial fold on the 

 brachial. Lower-Middle Cambric. 



