.TTTRASSIC FOSSILS. 347 



Formation and locaVitjf. — In rod sandstones of Jurassic a<^e, from 80 to 

 100 feet above tlie red beds referred to the Triassic, on the east side of 

 Spearfish Creek, near its junction witli tlic Kedwater, I^lack Hills, Dakota. 



RHYNCHONELLID^. 



Genus RHYNCHONELLA Fischer. 



KHYNCHONELLA MYRINA. 



Plate 3, ligs. 6, 7. 



Rhynclionella spA M. & H., Pal. Up. Missouri, p. 71, PI. 4, fig. 3. 



RhynchoneUa myrina H. & W., Geol. Expl. Fortieth Parallel, vol. iv, p. 284, PI. 7, figs. 1-5. 



Shell of moderate size, subcircular or subtriangular in general outline, 

 with moderately convex valves. Ventral valve depressed-convex in the 

 middle portions, more sharply rounding upward along the cardinal slope, 

 and marked by a broad, shallow depression along the middle in the anterior 

 half of the shell, becoming somewhat more abrubtly bent upward in front; 

 beak moderately large, pointed, and incurved. Dorsal valve more convex 

 than the opposite and more generally rounded, the beak appressed and 

 incurved within that of the ventral ; middle portion of the valve elevated 

 forward of the center, forming a broad mesial elevation corresponding to 

 the depression of the other valve. 



Surface marked by from twenty to twenty-eight distinct, rather angu- 

 lar, radiating plications, from four to eight of which are depressed in the 

 sinus of the ventral valve, and a corresponding number elevated on the fold 

 of the dorsal. These are crossed by distinct, often strongly-marked, con- 

 centric lines of growth and finer concentric striae, which arch across the 

 plications more or less abruptly according as they are more or less angular. 



The shells under consideration are doubtless the same as those referred 

 to by Messrs. Meek and Hayden loc. cit., and are also identical with those 

 described under the name JR. myrina H. & W. loc. cit., but are quite distinct 

 from those given under the name of B. gnatliopliora Meek, Geological Sur- 

 ve}' California, Pal., Vol. I, Plate 8, Fig. 1, being proportionally broader, 

 less equally convex, and much more finely plicated. 



Formation and locality. — In white limestones of Jurassic age, above the 

 red beds referred to the Triassic, at Red Canon Creek, Black Hills, Dakota. 



