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



forms more than one-sixth. Of course the specimens are more or less 

 variable in these characters, but the two forms can always be readily 

 distinguished when good examples can be had for comparison. 



"In its more arcuate form our shell agrees more nearly with Modiola 

 ornata Gabb, from the Cretaceous rocks of California; but that shell 

 differs very markedly in having its beaks decidedly less nearly termi- 

 nal, and a more decided and much more prominent lobe in front of 

 them. Another important difference is to be observed in the radiating 

 striae, which, on the anterior side of our shell are very minute and 

 closely crowded, while on that part of Mr. Gabb's species they are as 

 large and distant from each other as on any other part of the valves." 



Whiteaves states that Modiola tenuisculpata is very closely related to 

 Meek's species, and as it comes from approximately the same horizon 

 in Manitoba it will probably prove to be a synonym. 



Locality and position. — The types came from the lowest fossiliferous 

 bed of the Cretaceous section at Coalville, Utah, where it is abundant. 

 It also occurs at a much higher horizon in the "third ridge' 7 of the 

 same section and at several localities in southern Utah. It is common 

 in the Colorado Cretaceous beds at Bear River city, Wyoming, and the 

 same or a closely related species has been found in the Bear River for- 

 mation on Twin creek, Wyoming. 



PINNID^E. 



Genus PINNA Linnaeus. 



Pinna petrina White. 



PI. xix, Fig. 4; PI. xx, Pig. 1- 



Pinnapetrina White, 1874, Expl. & Sur. West of 100th Meridian, Prelim. Rept. Invert. 



Foss., p. 24; 1876, U. S. Geog. & Geol. Sur. West 100th Meridian, vol. iv, p. 182, PI. 



13, Figs, la and b. 

 Pinna stevensoni White, 1880, Proc. U. S. Nat. Mus., vol. in, p. 47. 



Revised description : 



" Shell moderately large, broad, rather thick, rapidly expanding in 

 height as it increases in length 5 dorsal margin concave; ventral mar- 

 gin convex; a more or less strongly-raised carina extending from the 

 beak to the posterior margin, defining a prominent longitudinal angle 

 along the median portion of each valve, which is placed a little nearer 

 the ventral than the dorsal border; transverse section rhomboidal, the 

 sides of the rhomb slightly convex; posterior margin oblique with the 

 axis of the shell, forming a distinct but obtuse angle with the dorsal 

 margin. The acute angle which it would form with the ventral margin 

 if continued all the way to it in a direct course is abruptly rounded. 



" Surface marked by strong, distinct lines of growth, which run ob- 

 liquely downward and* backward in a nearly direct course from the dor- 

 sal margin, across the mesial angle, to near the ventral margin, where 



