OBOLID^. 475 



As determined by a studj^ of interior casts, the plane of the cardinal area of the ventral 

 valve coincides near its edges with the edge of the shell, rising slightlj- toward the deep, rounded 

 pedicle furrow. The area is divided at the center by the pedicle furrow, and toward its margms 

 by elevated lines, or extremely narrow ridges, that extend from the apex with a slight^ outward 

 curvature to the base of the area (the flexure marks of the area parallel with its base) ; they 

 are rounded and strong in the pedicle groove, sharp and crowded on the lateral slopes; they 

 cross the pedicle groove transversely, slope forward across the central spaces (splanchnocoele 

 of Mckwitz [1896, p. 116]) curve almost backward in the flexure lines and slope forward at a 

 low angle across the lateral spaces (pleuroccele of Mickwitz [1896, p. 116]). The area forms a 

 thin shelf between the pedicle groove and the lateral margins, the undercut extending far back 

 under the area. This is sho\\Ti in the cast by a tliin projection of the embedding rock over the 

 area (PL XVII, figs la, Ic, and If), where the broken edges remain. The area of the dorsal 

 valve is lower and less prominent; the stride of growth cross it parallel to the anterior margin, 

 being interrupted by the flexure lines, which are but slightly indicated on most specimens. 

 The shaUow furrow, corresponding to the pedicle furrow of the ventral valve, usually extends 

 across the space between the flexure lines. The pedicle groove on the ventral valve is deep, 

 narrow, and strongly defined, tapering very graduall}^ from the anterior end toward the beak. 



The cast of the visceral cavity (v), including the heart-shaped pit (x) of the ventral valve, 

 is shown by Plate XVII, figures la, Ic, If, and Ig, the latter particular^ well by figure If, 

 where it strongly resembles the same space in Oholus apoUinis Eichwald. There are no traces 

 of a median septum in the ventral valve; in the dorsal valve it is sho'WTi in the casts as a very 

 narrow, clearly defined, sharp depression (s) extending a short distance back of the central 

 muscle scars, in some specimens, and forward nearly to the anterior lateral scars (PI. XVII, 

 figs. Ih and Ij). 



The muscle scars are finely shown m casts of both valves. The umbonal scar (gg) of the 

 ventral valve is divided as in Oholus, the pedicle scar (m) being situated between the two parts 

 (PI. XVII, fig. If) ; the latter are oval in outlme, their longer axis being slightly inclined forward 

 and outward. In the dorsal valve the umbonal scar (g) is very close to the area and extends 

 across nearly the entire distance between the central portion of the area delimited by the flexure 

 lines (PL XA1I, figs. Ih and Ij). In one specimen fine strise cross the scar subparallel to the 

 median line of the shell. 



The scars of the central muscles are not clearly defined in the ventral valve, owing to their 

 being crowded in with the scars of the middle and outside laterals. Their position, judging 

 from the location of the centrals in Oholus, is within the trapezoidal area (c) indicated on Plate 

 XVII, figures If and Ig. The central muscle scars (hh) are large and distincth^ defined on a 

 number of casts of the interior of the dorsal valve. They are situated a little in advance of the 

 center of the sheU, on each side of the median septum, with their longer axis mclined outward 

 at an angle of nearly 45° (PL XVII, figs. Ih and Ij). The anterior laterals (j) are clearly defined 

 in both valves; their form, size, and position are shown by figures If and Ig for the ventral 

 valve, and figures Ih and Ij for the dorsal valve. The middle and outside lateral scars on the 

 ventral valve are situated in the trapezoidal area (c), but neither are clearly separable in an}' 

 of the specimens in the collection unless it be in that represented by figure Ic, where a small 

 round scar (k) occurs, corresponding in position to that of the middle lateral scars of Oholus. 

 On the dorsal valve the middle and outside laterals are closely united as in Oholus, and they 

 are apparently combined in the elongate scar (1) (PL XVII, figs. Ih and Ij). This scar corre- 

 sponds m position with the two scars in Oholus, in which genus it is usually impossible to dis- 

 cover the line of demarcation between them. The transmedian scars (i) are distinctly shown 

 on the dorsal valve (PL XVII, figs. Ih and Ij), but in the ventral valve they are so closely 

 united with the anterior laterals (j) that it is impossible to differentiate them. 



Portions of the markings left on the sheU hj the vascular system are finely preserved in a 

 few casts of the interior of the valves. The desci'iption is of the present appearance of the 

 casts, which is the reverse of the actual condition in the interior of the shell. The main or trunk 



