76 COLORADO FORMATION AND ITS INVERTEBRATE FAUNA, [bull. 106. 



Genus INOCERAMUS Sowerby. 



Inoceramus fragilis H. and M. 



PI. xi, Figs. 1-5. 



Inoceramus fragilis Hall and Meek, 1856, Mom. Am. Acad. Arts, and Sci. n. s., vol. v, 

 p. 388, PL 2, Fig. 6a and b; White, J876, U. S. Geog. and Geol. Sur. west of 100th 

 Meridian, vol. iv, p. 178, PL 15, Fig. 3; Meek, 1876, U. S. Geol. Sur. Terr., vol. 

 ix, p. 42, Figs. 1 and 2 in text and PL 5, Fig. 5; Meek, 1876, Macomb's Expedi- 

 tion from Santa F6 to junction of Grand and Green rivers, Geol. Rept., p. 127, 

 PL 1, Fig. 6; Whitfield, 1880, Geol. Black Hills of Dakota, p. 390, PL 9, Fig. 10. 



Inoceramus howelli White, 1876, Geol. Uinta Mts., p. 114; 1879, 11th Ann. Rept. U. S. 

 Geol. Sur. Terr., p. 284, PI. 4, Figs, la, b, c. 



Inoceramus perplexus Whitfield, 1880, Geol. Black Hills of Dakota, p. 392, PL 8, Fig. 

 3, ami PL 10, Figs. 4 and 5. 



The original description and figures were taken from a small imper- 

 fect specimen in which the anterior border was mistaken for the hinge 

 line. After studying better material from the same horizon and in part 

 from the typical locality, Prof. Meek drew up the following description : 



" Shell thin, broad-subovate, higher than long, moderately convex, 

 subequi valve; anterior side vertically truncate from the beaks with a 

 slightly concave outline; basal and posterior borders forming a more 

 or less regular, nearly semicircular curve; hinge line rather short, and 

 standing nearly at right angles to the truncate anterior. Beaks pointed 

 equal, scarcely rising above the hinge, curving inward and slightly for- 

 ward at the points. Surface marked by fine lines of growth, and a few 

 obscure traces of concentric undulation. 



"Height, about 1.43 inches; length, 1.07 inches." 



The study of a considerable number of good specimens from Upper 

 Kanab valley, southern Utah, and from Huerfano park, Colorado, in 

 connection with the types of Inoceramus perplexus and I. hoicelli and 

 the specimens of I. fragilis figured by Meek, have convinced me that 

 these all belong to one variable species. The differences observed, 

 however, are partly due to differences in the state of preservation. In 

 some individuals the surface is almost smooth, even when the shell 

 has attained considerable size, while others, such as the types of I. per- 

 plexus, are marked with very distinct concentric ridges. The variety to 

 which the name I. howelli was given differs from the typical form in 

 being slightly more convex and more inequivalve, in having a distinct 

 posterior auriculation and a broad, shallow furrow on each valve ex- 

 tending from the umbonal region to the posterobasal margin, where it 

 forms a slight emargination. Nearly all of the specimens from Huer- 

 fano park, Colorado, might be referred to this variety, but none of the 

 features on which it is based are constant, so that in a large series 

 every gradation between it and the typical form may be found. 



Locality and position. — The original type came from near the mouth of 

 Vermilion river, on the Missouri, at or near the base of the Fort Benton 



