OBOLELLIDiE. 587 



Obolella Billings, Mickwitz, 1896, M6m. Acad. imp. sci. St.-P6tersbourg, 8th ser., vol. 4, No. 2, pp. 116, 121, and 

 123-126. (Copies the description of the genus as given by Hall and Clarke, 1892c, pp. 66-67, on pp. 123 and 

 124 and gives a general discussion of the generic relations of Obolella and Obolus, etc.) 

 Obolella Billings, Grabau and Shimbr, 1907, North American Index Fossils, vol. 1, p. 188. (Described.) 

 Obolella Billings, Walcott, 1908, Smithsonian Misc. Coll., vol. 53, No. 4, PL XI, and pp. 142 and 145. (Classifica- 

 tion of genus.) 



Generic characters. — Shell subequivalve, moderately convex; dorsal valve more elevated at 

 the umbo than the ventral; longitudinally ovate or oval in outline, with the dorsal valve some- 

 times transversely oval. Surface marked by concentric strise of growth and radiating strije, 

 except in the subgenus Glyptias, which has irregular transverse lines crossing the concentric 

 striae. 



Shell substance unknown in an unaltered condition; it is now calcareous but it may have 

 been calcareocorneous as in Oholus. Shell structure formed of a thin outer layer with many 

 thin inner layers or lamellae more or less oblique to the outer layer; the short lamellae of the 

 anterior portion of the valves are more oblique than the longer lamellae of the central and 

 posterior portions and lie at the edges in the same plane as the margins of the valves. 



Area of the ventral valve rises from the plane of the margins of the valve at angles vary- 

 ing from 10° to 70°; broadly subtriangular when the beak is projecting, otherwise rounded at 

 the posterior margin almost as much as the area of the dorsal valve; the latter is usually on 

 the plane of the margins of the valve and broadly rounded posteriorly; both areas are striated 

 parallel to the base; in the ventral valve a narrow, shallow furrow divides the area midway; 

 the area of the dorsal valve is divided by a narrow, raised, triangular space, bordered on 

 either side by a more or less deeply impressed groove formed by a narrow fold of the area 

 lamellae; outside of this groove, and between it and the outer flexure fold of the area (f), a 

 depressed triangular space occurs that has led authors to consider it as the scar of the point 

 of attachment of cardinal muscles (PL LIV, figs. Ih, li). It sometimes occurs that the inner 

 angle of the area of the ventral valve adjoining the pedicle opening projects forward so as to 

 form a toothlike knob that in the cast is shown by a deep indentation beside the cast of the 

 pedicle opening and between the latter and the projecting cast of the undercut beneath the 

 area (PI. LV, figs. If, Ig). Pedicle opening or foramen of the ventral valve cylindroconical, 

 broad at the base and tapering to a minute apex; position of the external opening a little in 

 advance of the point of the beak; as indicated by casts of the tube in 0. atlantica and a speci- 

 men of 0. crassa. The opening varies in proportional size in different species; it is usually 

 marked by rather strong striae of growth. It extends upward and backward through the shell, 

 very much as does the foramen in Trematoholus and Botsfordia. 



Splanchnocoele " of the ventral valve confined to the posterior half of the valve while in 

 the dorsal valve it extends forward to and in some instances beyond the center; in both valves 

 it extends back to the central part of the area which is bounded by the flexure lines (PI. LIV, 

 figs, li, 2g, 2m). Traces of a median septum are shown in the dorsal valve (PI. LIV, figs. Ih, 

 li, and PI. LV, fig. li), but no definite septum has been observed in the ventral valve. A 

 central median ridge of varying degrees of size and length often extends toward the central 

 portion of the dorsal valve; when it is large, a deep rounded groove usually occurs on each 

 side of it on the inner slope of which the central muscle scars may be situated (PI. LIV, figs. 

 2k-m). Often the median ridge is practically absent (PL LV, figs, li, 4a). 



The grooves of the main vascular sinuses begin in each valve at the front margin of the 

 area near the median line, and in the ventral valve gradually extend forward and outward 

 toward the front half of the shell where they begin gently to curve inward, terminating toward 

 the front of the valve, the distance and curvature varying in different species (PL LIV, figs. 

 If, 2g, 2h, and PL LV, figs. If, Ih, 3c); in the dorsal valve the main vascular sinuses curve 

 out more rapidly (PL LIV, figs, li, 2m) and are much less prominent; none of the secondary 

 radial canals or the peripheral vascular sinuses have been observed. The course of the 

 parietal scar between the main vascular sinuses is in front of the splanchnocoele in the ventral 



» Using nomenclature of Miclcwitz, 1896. 



