OBOLELLID^. 589 



The specimens occur in a compact, bkiish-gray oolitic limestone. None of them show the 

 area or interior of the valves. The reference to Oholella is based on the general form and calca- 

 reous shell. The shells vary in size from 3 to 5 mm. 



Oholella asiatica may be compared with young shells of 0. crassa. Its broadly elliptical 

 form and slight convexity distinguish it from other species of the genus. 



It may be that if material is found showing the interior valves' the generic reference will be 

 changed; but with the data now available, the reference is to Oholella. 



The geological horizon is in some doubt, as the specimens were found in a block of river 

 drift limestone. The associated fragments of trilobites are too indefinite for determination. 

 Oholella is a Lower Cambrian genus as far as known, and other blocks of river drift limestone 

 at the same locality contained fragments of Redlichia, so the reference of the species is made to 

 the Lower Cambrian. By error 0. asiatica was included in the list of the Middle Cambrian 

 fossils of China [Walcott, 1905b, p. 5]. 



Formation and locality. — Lower Cambrian: (C32') Limestone bowlder collected in river drift 1 -mile {1.6 km.) 

 south of Chonpinghien, on Nankiang River, southern Shensi; and (C17) ferruginous limestone nodules in the brown 

 sandy shales at the top of the Manto shale [Blackwelder, 1907a, p. 27 (list of fossils at top of page) and fig. 6 (bed 15), 

 p. 25], at Changhia, Shantung; both in China. 



Obolella atlantica Walcott. 



Text figure 51, page 590; Plate LV, figures 1, la-i. 



Obolella crassa Shaler and Foerste [not (Hall)], 1888, Bull. Mus. Comp. Zool. Harvard Coll., whole ser. 16, No. 2 

 (geol. ser. 2), p. 27, PI. I, figs. la-f. (Described.) 



Obolella sp. Shaler and Foerste, 1888, idem, pp. 27-28, PL I, figs. 2a-c. (Described.) 

 ■- Obolella atlantica Walcott, 1890, Proc. U. S. Nat. Mus. for 1S89, vol. 12, p. 36. (Characterized.) 



Obolella atlantica Walcott, 1891, Tenth Ann. Kept. U. S. Geol. Survey, p. 611, PI. LXXI, figs. 1, la-c. (Copies Wal- 

 cott, 1890, p. 36. Figs. 1 and lb are copied in this monograph, PL LV, figs, lb and Ic, respectively. Figs, la 

 and Ic were slightly altered before they were used in this monograph, PL LV, figs, li and Ih, respectively.) 



Obolella atlantica Walcott, Matthew, 1899, Trans. Roy. Soc. Canada for 1899, 2d ser., vol. 5, sec. 4, No. 3, p. 70. (Copies 

 Walcott, 1890, p. 36, and discusses species.) 



Obolella atlantica Walcott, Burr, 1900, American Geologist, vol. 25, p. 47. (Discussed.) 



Obolella atlantica Walcott, Grabau, 1900, Occas. Papers, Boston Soc. Nat. Hist., No. 4, vol. 1, pt. 3, pp. 620-621, PL 

 XXXIV, figs. 3a-b. (Described.) 



Obolella crassa Shaler and Foerste, Gorham [not (Hall)], 1905, Bull. Roger Williams Park Museum, No. 9, PL I, figs. 

 la-f. (No text reference. Figs, la-f are copied from Shaler and Foerste, 1888, PL I, figs, la-f.) 



Obolella atlantica Walcott, Gorham, 1905, idem, PL I, figs. 2a-c. (No text reference. Figs. 2a-c are copied from Shaler 

 and Foerste, 1888, PL I, figs. 2a-c, respectively.) 



Obolella atlantica Walcott, Grabau and Shimer, 1907, North American Index Fossils, vol. 1, p. 188, figs. 222a-d. 

 (Described. Figs. 222a-d are copied from Walcott, 1891a, PL LXXI, figs. 1, la-c, respectively.) 



General form ovate, with ventral valve obtusely acuminate in the uncompressed speci- 

 mens; dorsal valve nearly circular, with the transverse diameter the greatest in the best pre- 

 served shells (PL LV, figs. Ih and li). Valves moderately convex. Surface of the shell marked 

 by concentric strise and more or less imbricating lines of growth; fine radiating striiB that are 

 sometimes raised lines strongly mark most shells, although in some instances the stripe are absent, 

 probably from having been worn away by abrasion. Wlaen the imbricating lamelhe of the shell 

 are broken the surface has the appearance shown by figure Ic. 



The shells from the limestones of Manuels Brook appear to be relatively thick and formed 

 of several layers, the lamellae being arranged so as to terminate slightly oblique to the outer 

 surface, thus forming the foundation for the imbricating lines of growth. The Smith Point 

 material shows only casts of the outer and inner surfaces, as do most of the specimens from 

 North Attleboro. The thickening of the outer portions of the shell by the growth of the 

 oblique lamellse forms a strong interior margin which is usually flat (PI. LV, figs, la and Ih), 

 but it is sometimes oblique to the plane of the edges of the valves (fig. la). The ventral valves 

 average about 5 mm. in length, the largest is 8 mm. ; the dorsal valve is a little shorter than the 

 ventral. 



