356 • EXPLORATIONS ACROSS THE GREAT BASIN OF UTAH. 



however, is thinner, and differs in being- without imbricating marks of growth, while 

 its plications are smaller. In addition to this, the shell figured by Mr. Marcou is now 

 known to be a Cretaceous species, that holds a position far above the horizon from 



This species is named in honor of Mr. Henry Engelmann, of Saint Louis, Geologist 

 of Captain Simpson's expedition. 



Locality and position.— Jurassic beds at Red Buttes, on the North Platte, lati- 

 tude 42 50', longitude 106° 40' west. 



Geyph^a caloeola, Quenstedt ?. 



Several specimens i indistinguishable from the species cited above were obtained 

 from the Jurassic beds near the Red Buttes, on the North Platte. The specimen fig- 

 ured has the form and other characters of a true Gryphma; but some of the others have 

 the whole beak truncated, and present more the appearance of Oystrea; though there 

 seem to be intermediate gradations between these forms. They show clearly the radi- 

 ating stria? seen on the under valve of G. calceola, as known in Europe. 



Genus CAMPTONECTES, Agassiz. 

 Camptonectes bellistriata, Meek. 



Plate 3, fig. 3, a, b, c, d. 



Pech-n bellistriata. Meek (July, 1860). Proceed. Acad. Nat. Sci. Philad., XII, 311. 



Camptonertes belihtriaUt. Merfc (1^64), Smithsonian Check-List N. Am., Juras. Fossils, 28; and (1865) Paheont. Upper 

 Mo., 77, wood-cuts A, B, C. 



Shell of medium size, subeircular, sometimes wider than long, thin, compressed, 

 nearly or quite equivalve; hinge straight and very short ; posterior wing small or 

 nearly obsolete, obliquely truncated ; anterior wing small, vertically truncated at the 

 extremity, and in the right valve separated from the margin below by a distinct more 

 or less angular sinus, from which a shallow fiat groove extends obliquely to the beak; 

 beaks of both valves small, and rather compressed; surface ornamented by numerous 

 fine, arched, bifurcating strae, crossed by extremely small, closely arranged concen- 

 tric lines, which are often nearly obsolete on the radiating striae over the more convex 

 portions of the valves, but quite distinct in the slender depressions between, to which 

 they impart a punctate appearance. 



Length (broad variety), 2.26 inches; breadth, 2.65 inches; convexity, 0.64 inch. 



The radiating striae, of which about six to seven may be counted in the space of 

 one-tenth of an inch near the border on the middle of the valves, are more crowded on 

 the lateral margins, where they curve strongly outward. They are separated by exceed- 

 ingly delicate impressed lines, and on some parts of the shell occasionally present the 

 peculiarity of bifurcating, and again coalescing at intervals. On the lateral margins the 



