MESOZOIO FOSSILS. 619 



River. The type came from Spring Canyon and lower canyon of the Yel- 

 lowstone, Montana. 



Trigonia elegantissima Meek. 



PL LXXIII, fig. 2. 



Trigonia elegantissima Meek, 1873: Ann. Kept. U. S. Geol. Surv. Terr, for 1872, p. 474. 



Shell small, subtrigonal in outline, moderately convex, with prominent, 

 acute, recurved beaks; posterior umbonal ridge prominent, angular, and 

 curved; anterior end broadly rounded; posterior end subangular below at 

 the extremity of the umbonal ridge and forming a convex slope to the beak 

 above; escutcheon not distinctly marked; posterior area depressed and 

 bearing numerous equal, fine, radiating lines; remainder of surface with 

 regular, closely arranged, small concentric ribs that show a tendency to 

 bend downward toward the front. 



Length of figured specimen, 21 mm.; height, 11 mm.; convexity of 

 single valve, 4 mm. 



This species is closely related to T. americana, from which it differs in 

 outline, and more especially in having much smaller and more numerous 

 concentric ribs. In specimens of T. americana no larger than the type of 

 this species the spaces between the ribs are at least a millimeter wide. 



Meek's original description, given in a footnote to the list of fossils 

 from Devils Slide, Cinnabar Mountain, Montana, is as follows: "A small 

 species of the type of T. costata, but having- the concentric or horizontal 

 costse on the sides of the valves very delicate, closely arranged, and but 

 slightly larger than the radiating ones on the posterior dorsal region, or 

 corselet. The valves are rather compressed, about one-fourth longer than 

 wide, and have the posterior umbonal slopes acutely angular." A single 

 valve corresponding to this description, but not labeled, is in the original 

 collection from Cinnabar Mountain studied by Meek, and this is probably 

 his type. The specimen figured was collected at the same place by Mr. 

 W. H. Weed. 



Trigonia montanaensis Meek. 



Trigonia montanaensis Meek, 1873: Ann. Rept. U. S. Geol. Surv. Terr, for 1872, p. 472. 

 White, 1880: Idem for 187S, p. 247, PL XXXVIII, fig. 2a. 



The types are from the locality near the lower canyon of the Yellow- 

 stone. A few specimens were obtained 1 mile from Swan Lake Valley, 



