64 RESEARCH IN CHINA. 



Genus OBOLUS Eichwald. 



For discussion of the genus Obolus see Monogr. U. S. Geol. Survey, vol. LI, 

 1912, pp. 370-380, text-figure 34, p. 374. 



Obolus chinensis (Walcott). 

 Plate i. Figures n, na-c. 



Obolus (Lingulella) chinensis WALCOTT, 1905, Proc. U. S. Nat. Mus., vol. xxviu, p. 328. (Species 



described and discussed as below.) 

 Obolus chinensis (WALCOTT), 1912, Monogr. U.S. Geol. Survey, vol. LI, p. 387, plate xxxix, figs. 7, -;a-b. 



(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 striae 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 striae in addition to concentric striae; the 

 inner surface shows a few scattered punctae in addition to radiating and concentric 

 striae outside the visceral area. The shell is of medium thickness and formed of 

 a thin outer layer and several inner layers or lamellae that are slightly oblique to 

 the outer layer; the lamellae 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 shantungcnsis 

 [p. 76]. It is closely related in form to Obolus prindlei (Walcott) [1912, plate xxvn, 

 figs- 3> 3 a ~ e ] of the upper OlencUns 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 0. prindlei. With the data available for comparison it 

 differs from 0. prindlei in its shorter cardinal area and visceral area in the dorsal 

 valve. From Obolus damesi [p. 65] it differs in 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. 



