64 RESEARCH IN CHINA. 
Genus OBOLUS Eichwald. 
For discussion of the genus Obolus see Monogr. U. S. Geol. Survey, vol. 11, 
1912, pp. 370-380, text-figure 34, p. 374. 
Obolus chinensis (Walcott). 
Plate 1, Figures 11, 11a—c. 
Obolus (Lingulella) chinensis WaLcoTT, 1905, Proc. U. S. Nat. Mus., vol. xxv, p. 328. (Species 
described and discussed as below.) 
Obolus chinensis (WaLcoTT), 1912, Monogr. U.S. Geol. Survey, vol. L1, p. 387, plate xxx1x, figs. 7, 7a—). 
(A copy of the preceding reference.) 
General form broadly subovate, with the ventral valve broadly subacuminate 
and the dorsal valve oval, with the postero-lateral slopes somewhat straightened. 
There is some variation in the proportions of width and length in different shells. 
Valves rather strongly convex for so thin a shell, in this respect resembling species 
of Dicellomus. Surface marked by fine, concentric strie and lines of growth and 
very fine, undulating, concentric lines; this outer surface is usually a dull black 
and adheres to the matrix; when the outer layer is exfoliated the surface is shiny 
black and marked by numerous radiating striz in addition to concentric strie; the 
inner surface shows a few scattered puncte in addition to radiating and concentric 
striz outside the visceral area. The shell is of medium thickness and formed of 
a thin outer layer and several inner layers or lamelle that are slightly oblique to 
the outer layer; the lamella are more numerous and more oblique to the outer 
layer over the anterior and antero-lateral portions of the shell. The ventral valves 
average 3 mm. in length, with width varying from 2 mm. to 2.5 mm.; the dorsal valve 
is slightly shorter than the ventral, when the two valves have the same width. 
The interior of the ventral valve shows that the area is short and divided 
midway by a narrow pedicle furrow that merges into the visceral area, which extends 
forward about one-third the length of the valve; on each side of the anterior central 
portion of the visceral area there are two small trapezoidal areas within which the 
central scars and the middle and outside lateral muscle-scars were attached; the 
small transmedian and antero-lateral muscle-scars are situated close to the main 
vascular sinus opposite the anterior portion of the central visceral area. A partial 
cast of the interior of the dorsal valve shows a rather wide visceral area with the 
antero-lateral scars about the center of the valve and the central scars about one- 
fourth of the distance back from the center to the posterior margin. On the 
impressions left by the vascular system only the main vascular sinuses of the 
ventral valve have been seen; these are situated about midway between the central 
visceral area and the margins of the valve. 
This species occurs abundantly in association with Acrotreta shantungensis 
[p. 76]. Itis closely related in form to Obolus prindlei (Walcott) [1912, plate xxvur, 
figs. 3, 3a-e] of the upper Olenellus zone of eastern New York, and belongs with a 
group of small shells that have a wide vertical and geographical range, as is noted 
under the description of O. prindlet. With the data available for comparison it 
differs from O. prindlei in its shorter cardinal area and visceral area in the dorsal 
valve. From Obolus damesi |p. 65] it differsin being broader in proportion to its 
length. 
This species is fairly common in southwestern Manchuria in the Dorypyge 
richthofeni zone in association with Obolus damesi Walcott. I find that the two 
species are very closely related and that when the valves of O. damesi are flattened 
and broadened by compression it is difficult to decide to which species they should 
be referred. 
