23 



above the cardinal line, slightly enrolled and directed forward, situated 

 at about one-third of the length from the anterior extremity; antero- 

 cardinal line abruptly declining in front of , the beaks to the middle of 

 the height of the valve ; anterior end narrow and sharply rounded ; 

 posterior end very broadly rounded, and the postero-cardinal and basal 

 margins subparallel behind the beaks, the extreme height of the shell 

 continuing to the posterior end; antero-basal line strongly curving 

 upward to the anterior end of the shell. Surface of the valves appar- 

 ently smooth. 



The shell differs from any of the preceding species in the greater 

 length and proportionate size of the posterior portions of the shell, in 

 the situation of the beaks, and in their strong anterior inclination, the 

 others being much more erect than those of this species. It will be 

 readily recognized by the inflated valves, subparallel cardinal and basal 

 margins, and short anterior end. 



Formation and locality. In shaly limestones of the Jurassic formation 

 east of Belle Fourche, Black Hills, Dakota, associated with the preced- 

 ing species. 



FAM. V 



GENUS DOSINIA, Scopoli. 



DOSINIA JUEASSICA, H. sp. 

 Plate 5, figs. 21-24. 



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 an- 

 gular, 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 ventrieose, most gibbous on the utnbones and below, while an- 

 teriorly and posteriorly the shell is more compressed ; umbonal ridges 

 not defined. 



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

 concentric 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, submarginal ; 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 



