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



but not angular ; hinge margin shorter than the length of the valves 

 Surface ornamented by small, regular, simple, radiating costal, and 

 moderately distinct liues of growth." 



The costa) vary in number from thirty to thirty-five and three or four 

 of those on the posterior umbonal slope are more elevated, more angu- 

 lar, and farther apart than the others, giving that region of the shell 

 an angular appearance when they are not eroded. 



Some specimens are nearly circular in outline, the height and length 

 being equal, but in most cases the height from beak to ventral margin 

 is greater than the length or anteroposterior diameter. 



The specimens selected for illustration are somewhat smaller than 

 the average size. One from liuerfano park, Colorado, measures 14 mm 

 in length, 16™™ in height, and the convexity of the single valve is 6 mm . 

 A few examples have been collected that are considerably larger. 



Locality and position. — The types are from dark fissile shales near 

 the lower part of the Colorado formation, 20 miles west of Fort Bridger, 

 Wyoming. It is abundant at the same horizon on Sulphur creek, 

 Wyoming, and occurs in the coal-bearing sandstones at that locality. 

 The types of Gardium subcurtwn were collected in the sandstone of 

 the " second ridge" at Coalville, Utah, and it is found near the base of 

 the same section; also in the Pugnellus sandstone at several localities 

 in Huerfano park, Colorado. 



Cardium trite White. 



PL xxii, Figs. 7 and 8. 



Cardium trite While, 1879, Ann. Rept. U. S. Gcol. !5ur. Terr, for 1877, p. 291, PL 5, 

 Figs. 4a and b. 



Original description : 



"Shell broadly subovate or suborbicular, height and width about equal; 



valves gibbous, regularly arching from beak to base; median portion 

 regularly convex; sides a little llattened above the middle; rostral por- 

 tion narrowed, elevated, arched; beaks situated well toward the front, 

 much elevated above the hinge-line, prominent, incurved, approximate, 

 and turned very slightly, if any, forward; hinge-margin moderately 

 long for a species of this genus; front having a short, oblique trunca- 

 tion above, from the lower end of which the whole free margin of the 

 shell is continuously rounded to the posterior extremity of the hinge, 

 the convexity varying, but not very greatly, in different parts. Surface 

 marked by very numerous fine costal of nearly uniform size on all parts 

 of the shell, every third one of which only bears upon its back many 

 small nodes or short spines. 



"Height from base to beak, 35 m,n ; width, 36 mui . The height is, how- 

 ever, sometimes greater than the width. 



"This species bears more resemblance to C. curtum Meek & Hayden 

 than to any other species of Cardium yet described from the Cretaceous 



