CRUSTACEA— OSTRACOD A. 343 



hind, with slight concavity above; center of dorsal region faintly 

 impressed; surface finely reticulate. 

 Hamilton of New York, etc. 



VI. ScHMiDTELLA Ulrich. 



Shell 2 mm. or less in length, broadly subovate, most convex in 

 the dorsal region, right valve overlapping the left along the ventral 

 margin; neither eye tubercle nor sulcus. Ten species. Ordovicic 

 and Siluric. 



16. S. crassimarginata Ulrich. (Fig. 1656, o-g.) Ordovicic. 

 Center of valve strongly elevated, surrounded by marginal de- 

 pression. (Type of genus.) 



Stones River of Minnesota and Cincinnati region. 



17. S. umbonata Ulrich. (Fig. 1656, r-^.) Ordovicic. 

 More symmetrical than preceding, more strongly elevated dor- 

 sally with hinge line sunken. 



Black River of Minnesota. 



VII. Aparchites Jones. 



Shell not exceeding 3 mm. in length, equivalved, subovate or 

 oblong; hinge straight; ventral edge thickened, often beveled or 

 channelled; surface convex, mostly in the ventral half, smooth. 

 Genotype A. zvhiteavesi Jones. Over thirty species. Ordovicic 

 and Siluric. 



18. A. fimbriatus Ulrich. (Fig. 1657, &.) Ordovicic. 

 ^largin with blunt spines or fimbriae. 



Richmond of Minnesota. 



VIII. Paraparchites Ulrich and Bassler. 



Like Aparchites, but the right valve overlaps the left. Missis- 

 sippic-Permic. 



19. P. nicklesi Ulrich. (Fig. 1657, c-/.) Mississippic. 

 Hinge line prominent ; anterior end more oblique than posterior ; 



eye tubercle sharp ; convexity moderate. 

 Warsaw of Illinois, etc. 



