STANTON.] 



ARCID^. 93 



stones on East canyon creek, Wasatch range, Utah. The species has 

 been found near the base of the Cretaceous section at Coalville, Utah, 

 and it is common in the Pugnellus sandstone on Williams and Muddy 

 creeks, and in Poison canyon, Huerfano county, Colorado. 



Trigonarca depressa White (sp.) 



PI. xix, Fig. 2. 



Tdonearca depressa White, 1876, U. S. Gcog. and Geol. Sur. West lOOtli Meridian, vol. 

 iv, p. 183, PL 18, Figs. 13a, b. 



Original description: 



" Shell of moderate size, gibbous, irregularly trapezoidal or subovate 

 in marginal outline; posterior half of the basal margin nearly straight, 

 broadly rounding upward anteriorly, and sharply rounding the end of 

 the umbonal ridge to meet the posterior margin; anterior margin reg- 

 ularly rounded up to the hinge-line; posterior margin long, about equal 

 in length to the full height of the shell, nearly straight or slighly con- 

 vex, extending obliquely downward and backward; hinge line equal 

 to a little more than half the length of the shell; areas small, narrow, 

 well denned, slightly concave; beaks very small, depressed, a little 

 incurved; umbonal ridges very prominent, each bounding anteriorly a 

 flattened, three-sided space, along the middle of which there is a radi- 

 ating, raised line. 



" Surface marked by the usual lines of growth and also by numerous 

 small, flat, radiating costae of unequal width, with narrow, sharply im- 

 pressed interspaces between them ; costal largest upon and near the 

 umbonal ridge, becoming obsolete near the cardinal border, anteriorly 

 and posteriorly. 



"Length, measuring across at about midheight of the shell, 28 mm ; 

 height, from base to umbo, 23 mm ; thickness, both valves together, 

 about 20 mm . 



a This species is not fully represented in the collections, but its spe- 

 cific characters are very satisfactorily shown. A full collection of ex- 

 amples would probably show variations of outline, due to sex, in some 

 cases, whereby the aspect of the shell may be a little different from 

 that of the figure. It is perhaps as nearly related to I. Shumardi Meek 

 and Hayden as to any other described species, but it is clearly distin- 

 guished from that by its depressed beaks and prominent umbonal 

 ridges. 



" Position and locality. — Strata of the Cretaceous period; east bank 

 of Rio Puerco, 6 miles below Casa Salazan, New Mexico." 



The species associated with this at the typical locality belong to the 

 Colorado fauna. I have no doubt that it is generically related to Trig- 

 onarca obliqua and additional collections may show that it is still more 

 closely allied to that species. 



