OBOLELLIDiE. 593 



Obolella (Orbiculaf) crassa (Hall), Ford, 1871, Am. Jour. Sci., 3d ser., vol. 2, p. 33. (Merely changes generic refer- 

 ence.) 

 Obolella crassa (Hall), Billings, 1872, Canadian Naturalist, 2d ser., vol. 6, No. 2, p. 218. (New locality mentioned.) 

 Obolella desquamata (Hall), Billings, 1872, idem, p. 218, fig. 6, p. 217. (New locality mentioned.) 

 Obolella crassa (Hall), Billings, 1872, Am. Jour. Sci., 3d ser., vol. 3, p. 356. (Copy of Billings, 1872a, p. 218.) 

 Obolella desquamata (Hall), Billings, 1872, idem, p. 356, fig. 6, p. 355. (Copy of Billings, 1872a, p. 218, and fig. 6, 



p. 217.) 

 Dicellomus crassa Hall, 1873, Twenty-thii-d Ann. Rept. New York State Cab. Nat. Hist., p. 246, PI. XIII, figs. 6-9. 



(Generic reference changed.) 

 Obolella crassa (Hall), Ford, 1878, Am. Jour. Sci., 3d ser., vol. 15, p. 128. (Described and discussed.) 

 Obolella crassa (Hall), Ford, 1881, idem, vol. 21, pp. 131-133, figs. 1 and 2. (Described and discussed.) 

 Obolella chromatica (Hall), Walcott, 1885, idem, vol. 29, pp. 115 and 117; figs. 1 and 2, p. 116. (Mentioned in the 

 text. Figs. 1 and 2 are copied from Ford, 1881, figs. 1 and 2, respectively, p. 131. They represent Obolella 

 crassa and are wrongly referred to in the text, pp. 115 and 117, as Obolella chromatica.) 

 Obolella crassa (Hall), Walcott, 1885, idem, vol. 30, p. 21. (Correction of error on pp. 115 and 117 of preceding 



reference.) 

 Obolella crassa (Hall), Ford, 1886, idem, vol. 31, fig. 2, p. 466. (No text reference. Fig. 2 is drawn from the speci- 

 men figured by Ford, 1881, fig. 1, p. 131.) 

 Obolella crassa (Hall), Walcott, 1886, Bull. U. S. Geol. Survey No. 30, p. 114, PI. X, figs. 1, la-f. (Copies Hall, 

 1847, p. 290; Ford, 1878, p. 128; and Ford, 1881, pp. 131-133; and discusses species. Figs. Id and If are drawn 

 from the specimens figured by Ford, 1881, figs. 2 and 1, respectively, p. 131. The specimen represented by 

 fig. Ic is redrawn in this monograph, PL LIV, fig. 21.) 

 Obolella crassa (Hall), Walcott, 1891, Tenth Ann. Rept. U. S. Geol. Sm-vey, p. 612, PL LXXI, figs. 4, 4a-f. (New 



locality mentioned. Figs. 4, 4a-f are copied from Walcott, 1886b, PI. X, figs. 1, la-f, respectively.) 

 Obolella circe? Walcott [not Billings], 1891, idem, fig. 62, p. 611. (No text reference.) 

 Obolella crassa (Hall), Hall and Clarke, 1892, Eleventh Ann. Rept. State Geologist New York for 1891, PL III, figs. 



5-7. (No text reference.) 

 Obolella crassa (Hall), Hall and Clarke, 1892, Nat. Hist. New York, Paleontology, vol. 8, pt. 1, p. 70, PL II, figs. 31-36. 

 (Discussed. Fig. 33 is drawn from the specimen figured by Hall, 1847, PL LXXX, figs. 3a-b. Figs. 32, 36, 

 and 34 are copied from figs. 5-7, respectively, of the preceding reference.) 

 Obolella crassa (Hall)?, Grabau, 1900, Occas. Papers Boston Soc. Nat. Hist., No. 4, vol. 1, pt. 3, pp. 619-620, PL XXXI, 



fig. 3. (Described.) 

 Obolella crassa (Hall), Walcott, 1905, Proc. U. S. Nat. Mus., vol. 28, p. 321. (Discussed.) 



Obolella crassa (Hall), Grabau and Shimer, 1907, North American Index Fossils, vol. 1, pp. 188-189, figs. 222e-g, 

 p. 188. (Described. Figs. 222e-g are copied from Walcott, 1886b, PL X, figs. 1, le, and Ic, respectively.) 



General form ovate to suborbicular, with the ventral valve bluntly acuminate in some 

 examples. Considerable variation in outline occurs, as is shown by Plate LIV, figures 2, 2a, 

 2b of the ventral valve, and 2k, 2 1 of the dorsal valve. Valves moderately convex, with the 

 beak of the ventral valve depressed below the greatest elevation of the surface and slightly 

 curved over the low area; the beak of the dorsal valve terminates at the posterior margin. 



Surface of the shell marked by rather strong, concentric strise and lines of growth that 

 interrupt the continuity of fine radiating lines; it is rarely that the surface is well preserved; 

 usually the shells are smooth as though they had been, abraded. Shell rather tliick, and formed 

 of several layers or lamellae that are more or less oblique to a thin outer layer; the short oblique 

 lamellas encircling the anterior portion of the valves form imbricating lines when the outer 

 surface is removed (PL LIV, fig. 2b). Very few shells show the shell structure; usually the 

 calcareous matter has so far replaced the origmal shell that there is only a solid calcareous, 

 granular, or calcite shell, as shown by the broken section (fig. 2f). The weathered surface 

 often has a fibrous appearance similar to that of the shell of Obolella chromatica Billings. 



The area of the ventral valve is usually short, but it varies considerably in this respect 

 (PI. LIV, figs. 2g, 2h, 2j). It overhangs the posterior margin at an angle varying from 4.5° to 70° 

 and extends well out onto the cardinal slopes; fine strise of growth parallel to the margin cross 

 the area and pass into and across the central furrow; a narrow furrow divides the area midway 

 (PI. LIV, figs. 2g, 2h, and 2j). The specimen represented by figure 2h has a roimded cardinal 

 area and a very shallow median furrow; a small pedicle opening is outlined beneath the 

 rounded area, but it is not shown in the illustration. A tendency of the inner point of the 

 area next to the furrow to thicken and project forward into the cavity of the shell is found 

 in several specimens, but it is not as pronounced as in Oholella atlantica Walcott. The area 

 62667°— VOL 51, pt 1—12 38 



