JUEASSIO FOSSILS. 373 



VENERID.^. 



Genus DOSINIA Scopoli. 

 DOSINIA JUEASSIGA. 



Plate 5, figs. 21-24. 

 Dosinia jurassica Whitf., Prelim. Eept. Pal. Black Hills, 1877, p. 23. 



Shell of moderate size, subcircular or very slightly transverse in form, 

 and for the genus quite ventricose, with rather large, prominent beaks. 

 Cardinal line arcuate, but less curved than the general circle of the shell, 

 while the anterior end is more sharply curved and slightly angular and the 

 posterior portion from the extremity of the hinge to the basal border very 

 slightly flattened or truncate. Beaks anterior to the middle of the valve 

 directed forward and approximate at their apices; lunule small and very 

 slightly impressed; ligament proportionally long, but narrow, the shell at 

 its margins slightly incurved. Body of the valve ventricose, most gibbous 

 on the umbones and below, while anteriorly and posteriorly the shell is 

 more compressed; umbonal ridges not defined. 



Surface of the shell smooth with a few obsolete or faintly marked con- 

 centric irregularities, but no defined varices; fine concentric lines of growth 

 are seen on the perfect shells under an ordinary magnifier. Inner margins 

 of the valves smooth; muscular impressions, large but faintly marked, sub- 

 marginal. Pallial line deeply sinuate, the apex of the sinus being directed 

 toward the beak of the shell. 



The shell is a well-marked species of Dosinia, so far as the characters 

 have been ascertained, but the hinge structure has not been seen, none of 

 the casts being in a condition to show the teeth. The casts are quite abun- 

 dant on weathered specimens, where they have apparently been subject to 

 the percolating action of mineral waters, which has partially destroyed 

 them. 



Formation and locality. — In Jurassic rocks at the top or the highest hills, 

 two miles south of the Belle Fourche, and at a horizon 350 feet above the 

 Triassic red beds, east of the Belle Fourche, and also on the east side of 

 Spearfish Creek, near junction with the Redwater, Black Hills. 



