3 so NORTH AMERICAN INDEX FOSSILS. ' 



anterior end; two moderate nodes in posterior half confluent with 

 surface. 



Stones River of Kentucky. 



43. D. macra Ulrich. (Fig. 1664, a-c.) Ordovicic. 

 Differs from D. crass'moda in that the marginal ridge in anterior 



half becomes obsolete, the short longitudinal one joining the pos- 

 terior half; anterior node larger. 



Stones River of Minnesota, Tennessee, etc. 



XXI. MooREA Jones and Kirkby. 



Very small, more or less oblong or ovate shells; valves com- 

 pressed-convex, the free edges bounded by a raised marginal ridge, 

 sometimes wanting along the ventral side; inner region flat or 

 gently convex, without nodes, sulcus or pit. Genotypes M. obesa 

 and M. tenuis J. and K. Ordovicic-Carbonic. 



44. M. bicornuta Ulrich. (Fig. 1660, /,/',w.) Devonic. 

 Marginal ridge only in posterior portion, crescentic; anterior 



end with two short spine-like processes. 



Hamilton group of New York, Ontario, etc. 



XXII. Strepula Jones and Holl. 



Suboblong shells, with rounded ends, valves sHghtly convex, 

 without sulcus, traversed by numerous, twisted, thin ridges or ribs. 

 Genotype S. concentrica J. and H. Five species. Siluric-Devonic. 



45. S. plantaris Jones. (Fig. 1660, w,n'.) Devonic. 

 Slipper-shaped ; anterior end with six short spines, posterior end 



pitted, twisted ridge depressed S-shaped. 

 Hamilton of New York. 



46. S. sigmoides Jones. (Fig. 1660, <?.) Devonic. 

 Anterior end acute; sigmoid ridge oblique, enclosed in and 



united with a ridge parallel to margin. 

 Hamilton of New York, etc. 



XXHI. OcTONARiA Jones. 



Similar to Moorea, but distinguished in having the surface of 

 the valves raised into a thin spiral or ring-like ridge, which in the 



I 



