FOSSILS OF THE JUEASSIO PERIOD. 289 



Genus EUMICROTIS Meek. 



EUMICROTIS CURTA. 

 Plate VII, fig. 24. 



Avicula curta Hall, Stans. Rept. Salt Lake, p. 412, pi. 2, fig. 1. 



Avicula {Monotis) tenuicostatus M. & H., Proc. A. N. Sci. Phil., 1858, p. 50. 



Monotis curta M. & H., Proc. A. N. Sci. Phil., 18G0, p. 418. 



Eumicrotis curta M. & H., Smithsonian Check-List N. Am. Invert. Foss., 1804. 



Uumicrotis curta M. & H., Pal. Upper Missouri, p. 81, pi. 3, fig. 10. 



Shell small, suborbicular or obliquely ovate, a little higher than long, 

 or height and length subequal; valves convex, the left one the most rotund. 

 Hinge-line short, compressed behind and forming a small, obtusely-angular 

 wing; anterior side very short, scarcely forming awing, the anterior margin 

 rounding nearly to the beak; posterior margin gradually sloping backward 

 from the extremity of the hinge to a point below the middle of the valve, 

 whence it is rather sharply rounded to the junction with the basal line. Base 

 slightly prolonged on the posterior side of the median line, giving a little 

 obliquity to the shell. Eight valve apparently less convex than the left. 

 Surface marked by distinct radiating lines or ribs, which are narrower 

 than the spaces between, and usually become obscure or obsolete before 

 reaching the beaks. On the right valve, the radii are much less strongly 

 marked, while the concentric striae become more distinct. 



Among a large number of specimens there are no right valves in a con- 

 dition to show the hinge-features or byssal notch, and we are therefore left 

 somewhat in doubt concerning these features. The shells appear to possess 

 all the essential characters of E. curta, but vary so much among themselves 

 as to give rise to some doubt. 



Formation and locality. — In calcareous beds of Jurassic age, above the 

 gypsum beds at Ashley Creek, Uinta Range, Utah. Collected by S. F. 

 Emmons, esq. 



Genus CAMPTONECTES Agassiz. 

 Camptonectes bellisteiatus Meek. V 



Plate VII, fig. 13. 



Camptonectes hellistriatus Meek, Pal. Upper Missouri (Smithsonian Contributions to 

 Knowledge), p. 77, figs. A-D. 



"Shell very thin, compressed-lenticular, suborbicular in outline; valves 



19 P R 



