354 GEOLOGY OF THE BLACK niLLS. 



strongly toward the line separating the auriculations from the body of the 

 shell. Also by finer concentric striae crossing the radii, which are lamellose 

 in their structure in the depressions between the ribs, with deep pits between 

 them, giving a punctate character to the interspaces. Right valve of the 

 species not observed. 



The shell differs from C. beUistriata M. & H. in the more elongate 

 form, greater size of the auriculations, and coarser striae of the left valve. 

 We have no doubt, in our own minds, in regard to the proper reference 

 of this species. The uniform small size of the type specimens is no reason 

 why larger individuals might not occur, and there are also large numbers 

 of individuals in the collection of the size of Fig. 5, and smaller, having 

 exactly the same features as the t} 7 pe. It has been suggested that it might 

 be C. iilatisstformis White (Geology of the Uinta Mountains, p. 93,) but 

 that species is said to have the outline of C. stygius White, which this cer- 

 tainly has not, 



Formation and locality. — In shaly limestones of Jurassic age about 

 twenty feet above the red beds, east of the Belle Fourche River, near Bear 

 Lodge, Black Hills. 



AVICULID^E. 



Genus PSEUDOMONOTIS Bronn. 



PSEUDOMONOTIS (EUMICROTIS) CUETA. 



Plate 3, figs. 20-25. 



Avicula curta Hall, 1852, Stansb. Eept. Great Salt Lake, p. 412, PI. 2, fig. 1. 

 Avicitla (Monotis) temdcostata M. & H., Proc. Acad. Nat. Sci. Phil., 1858, p. 50. 

 Monotis curia M. & H., Proc. Acad. Nat. Sci. Phil., 18C0, p. 418. 

 Eumicrotis curta (Hall) M., Smithsonian Check List Jurassic Foss., 1864. 

 Eumicrotis curta (Hall) M. & H., Pal. Upp. Missouri, p. 81, PI. 3, f. 10. 

 Eumicrotis curta (Hall) H. & W., Geol. Expl. Fortieth Parallel, p. 289. 

 Pseudomonitis (Eumicrotis) curta (Hall) Meek, Geol. Snrv. Mont. Idaho, Wyom. and 

 Utah, 1872, p. 473. 



Shell of moderate size, suborbicular in outline, very slightly oblique, 

 and nearly equivalve, generally a little higher than wide; beaks subcentral 

 or nearest to the anterior end ; hinge short, usually less than half the length 

 of the shell below, longest on the posterior side; left valve the most rotund 



