OBOLID^. 433 



shell. Over the central portion of a ventral valve 13 distinct layers may be counted, and at 

 the anterior edge of the same shell 15 of the oblique lamellae that are strongly marked by the 

 radiating striae. The valves vary somewhat m size, outline, and degree of convexity. An 

 unusually acuminate ventral valve has a length of 20 mm. and a width of 19 mm. A dorsal 

 valve 15 mm. in length has a width of 16 mm. 



Casts of the interior of the ventral valve show a rather narrow area and a strong pedicle 

 furrow which is broad at the base. The striae of growth on the area are arranged in an imbri- 

 cated manner, very much as on the area of Obolus prindlei (Walcott) . The area of the dorsal 

 valve is narrow and almost concealed beneath the slightly overarching posterior margin. 



The cast of the ventral valve (v) shows roughly the heart-shaped cavity (x) of Oholus 

 (PL XVI, fig. 2k). There are no traces of a median septum in the ventral valve; in the dorsal 

 valve the septum extends as a low ridge with a sharp crest between the central muscle scars 

 and forward, so as to divide the anterior lateral scars. 



The muscle scars are finely shown in both valves. The umbonal scar of the ventral valve 

 is divided, as m Oholus, the pedicle scar bemg situated between the two parts. In the dorsal 

 valve the umbonal scar is very close to the area and arches slightly backward. 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 central and outside laterals. Their position, judging from the location 

 of the centrals m Obolus, is within the transversely trapezoidal area (c) , indicated in Plate XVI, 

 figure 2j . On the dorsal valve the centrals are large and distinctly defined, on many casts of 

 the interior of the shell and on partly exfoliated specimens. They are situated near the center 

 or sometimes a short distance in advance of the center of the shell, on either side of the median 

 ridge ; they are oval in outline, the longer axis diverging slightly outward anteriorly. The 

 anterior laterals (j ) are clearly defined in both valves. In the ventral valve they are very close 

 to the transmedian, if not actually in contact with them (PI. XVI, fig. 2k) ; in the dorsal they 

 are small and situated on either side of the median ridge a short distance in advance of the 

 central scars (PI. XVI, fig. 2h). The middle and outside lateral scars on the ventral valve are 

 situated in the trapezoidal area (c), but neither is clearly separable in any specimen of the 

 collection, unless it be in that represented by Plate XVI, figure 2k, where the outside lateral scars 

 (1) and the middle lateral scars (k) may be present. In the dorsal valve of Obolus, the middle 

 (k) and the outside Q) laterals are closely united, and they are apparently combmed m an 

 elongate scar at (1) (PI. XVI, figs. 2a and 2g). This scar corresponds in position with the two 

 sears in Oholus and it is deeply impressed in the thick shell of this species, much more so than 

 the large central scars, which apparently were but slightly impressed or else were situated upon 

 elevations on the inner surface of the shell. Some specimens show the elevation and others 

 the depression for the same set of scars. The transmedian scars (i) are not so distinctly shown 

 on most of the specimens of the dorsal valve, as they appear to have been lost in connection 

 with the deep depression formed by the outside and middle laterals. In one specimen, however, 

 (PI. XVI, fig. 2a) they are fairly well defined. In the ventral valve they are closely united 

 with the anterior laterals (j), but can be distinguished from them. 



The markings left on the shell by the vascular system are well preserved in a few casts 

 of the interior of the valve. The main or trunk sinuses are strong and curved inward a short 

 distance in front of the visceral cavity. Numerous fine peripheral branches radiate toward 

 the margin and a few short lateral branches toward the visceral cavity. In a dorsal valve 

 the main sinuses curve in front of the visceral cavity, very much as in the ventral valve. The 

 interior lateral sinuses radiate from the main sinus in toward the visceral cavity, and the 

 peripheral branches extend upward toward the margms. In some specimens where the pro- 

 jections of the visceral cavity back of the central muscle scars unite with the main sinus there 

 is an apparent broadening of the sinus that produces a rather sharp angle opposite the central 

 scars. In the ventral valve the depressed space in front of the visceral cavity is marked by 

 narrow, radiating, and longitudinal spaces, which apparently were occupied by the anterior 

 portion of the viscera, as the narrow canals unite toward the heart-shaped cavity and the 

 62667°— VOL 51, FT 1—12 28 



