356 THE PALEONTOLOGY OF MINNESOTA. 



LUinubolus (?) parvus. 



DINOBOLUS (?) PARVUS Whitfield. 

 1882. Dinobolus (?) parvus WHITFIELD. Geology of Wisconsin, vol. iv, p. 347, pi. xxvii, tigs. 8-10. 



FIG. 27. Dinoblus (?) parvus Whitfleld. A, interior of the dorsal valve as seen in a gutta-percha 

 impression, x 2; B, profile view of a cast of the interior, with the thickness and form of the shell indicated 

 by the outer line, natural size; U, gutta-percha impression showing the internal characters of the ventral 

 valve, x 2; D, cardinal view of the cast of the interior, x 2. 



Description: Shell small for the genus, subcircular in outline, lenticular in 

 profile, with the dorsal side somewhat more convex than the other, and both more 

 ventricose posteriorly than anteriorly. Posterior margin broadly triangular, curving 

 rapidly in the lateral portion to the broadly rounded anterior edge. Each valve 

 with a narrow false cardinal area, that of the ventral valve larger and slightly 

 produced beyond the dorsal into a samll, acute and but little incurved beak, beneath 

 which there appears to have been a narrow, concave triangular depression. 



A gutta-percha impression of the dorsal side of a cast of the interior shows a 

 large concave muscular area or platform, oval in outline and produced anteriorly 

 into a narrow but slightly elevated mesial septum which terminates near the anter- 

 ior margin. Upon this platform, and occupying the greater portion of it, are the 

 progressive scars of the lateral muscles, except antero-medially where the anterior 

 scar is present; the posterior portion is occupied by two well-defined, parallel, narrow, 

 elongate elevations, separated by a narrow sinus, the front of which may be homo- 

 logous with the cardinal scars, and the latter with the anal depression of Obolella. 

 A crescent so characteristic of Dinobolus does not appear to have been developed in 

 either valve. Platform of the ventral valve but slightly elevated, subquadrate in 

 outline, widest anteriorly, occupied by broad progressive tracks of the lateral 

 muscles, and separated medially by the faintly elevated median scars. Anterior 

 edge of platform not excavated. Within the cardinal margin of the ventral valve 

 are two short, narrow, rapidly diverging ridges, probably cardinal scars, separated 

 by a broad but short pedicle muscle impression. 



