OBOLELLID^. 591 



Obolella chromatica Billings. 

 Plate LIV, figures 1, la-i. 



Obolella chromatica Billings, 1861, Geo!. Survey Canada, Paleozoic Fossils, vol. 1, pp. 7-8, figs. 7a-d. (Described 



and discussed as a new species.) 

 Obolella chromatica Billings, 1861, Report on the Geology of Vermont, vol. 2, p. 947, figs. 346a-d. (Text and figures 



copied from preceding reference.) 

 Obolella chromatica Billings, 1862, Report on the Economic Geology of Vermont, by Hager, p. 219, figs. 346a-d. (Text 



and figures copied from preceding reference.) 

 Obolella chromatica Billings, 1863, Geol. Survey Canada, Fifteenth Rept. Progress, p. 284, figs. 288a-d. (No text 



reference. Figs. 288a-d are copied from Billings, 1861b, p. 7, figs. 7a-d, respectively.) 

 Obolella chromatica Billings, Hall, 1863, Sixteenth Ann. Rept. New York State Cab. Nat. Hist., p. 132, text figure. 



(Mentioned. The text figure is copied from Billings, 1861b, p. 7, fig. 7c.) 

 Obolella chromatica Billings, Hall, 1867, Trans. Albany Inst., vol. 5, p. 110. (Text and figure copied from Hall, 1863, 



p. 132.) 

 Obolella chromatica Billings, 1876, Am. Jour. Sci., 3d ser., vol. 11, pp. 176-178, figs. 1, 2, 3, p. 176; and figs. 4a-d, 



p. 177. (Described and discussed. Figs. 4a-d are copied from Billings, 1861b, p. 7, figs. 7a-d, respectively.) 

 Obolella chromatica Billings, Ford, 1881, Am. Jour. Sci., 3d ser., vol. 21, p. 133, figs, 3 and 4. (Compared with 



0. crassa. Figs. 3 and 4 are drawn from the figures given by Billings, 1876, figs. 1 and 3, p. 176.) 

 Obolella chromatica Billings, Walcott, 1886, Bull. U. S. Geol. Survey No. 30, figs. 9 and 10, p. 110 and p. 112, PI. 



XI, figs. 1, la-b. (Copies Billings, 1861b, p. 7, and 1876, pp. 176-177. Figs. 9 and 10 are copied from Billings, 



1876, figs. 1 and 3, p. 176. The specimen represented by fig. lb is redrawn in this monograph, PI. LIV, fig. Ih.) 

 -Obolella circe Billings, Walcott (in part), 1886, idem, p. 118, PI. X, fig. 3 (not fig. 3a). (Original description, Billings, 



1872a, pp. 219-220, of Obolella circe copied. The specimen represented by fig. 3 is redrawn in this monograph, 



PL LIV, fig. li. Fig. 3a represents a specimen of Quebecia circe.) 

 - Obolella chromatica Billings, Walcott, 1891, Tenth Ann. Rept. U. S. Geol. Survey, p. 611, PL LXXI, figs. 2, 2a-b. 



(Mentioned. Figs. 2, 2a-b are copied from Walcott, 1886b, PI. XI, figs. 1, la-b, respectively.) 

 Obolella circe Billings, Walcott, 1891, idem, p. 611, PI. LXXI, fig. 3 (not fig. 3a). (Mentioned. Pig. 3 is copied 



from Walcott, 1886b, PL X, fig. 3. Fig. 3a represents a specimen of Quebecia circe.) 

 Obolella chromatica Billings, Hall and Clarke, 1892, Nat. Hist. New York, Paleontology, vol. 8, pt. 1, p. 67, figa. 



28 and 29. (Mentioned in the text. Figs. 28 and 29 are copied from Billings, 1876, figs. 1 and 3, respectively, 



p. 176.) 

 Obolella chromatica Billings, Walcott, 1899, Mon. U. S. Geol. Survey, vol. 32, pt. 2, p. 446. (Note on generic rela- 

 tions.) 

 Obolella chromatica Billings, Walcott, 1905, Proc. U. S. Nat. Mus., vol. 28, p. 313. (Copied from Walcott, 1899, p. 446.) 



General form ovate, with ventral valve obtusely acuminate; dorsal valve broadly rounded, 

 with a tendency to become subacuminate in most specimens. Valves rather strongly convex, 

 with the beak of the ventral valve raised above the plane of the posterior margin, but usually 

 below the greatest elevation of the valve. The beak is slightly incurved over the area of the 

 ventral valve, and that of the dorsal valve terminates at the posterior margin, or may be 

 very shghtly curved over it. 



Surface of the shell marked by concentric strise and lines of growth and fine radiating 

 striae. The latter are usually absent either from the exfoliation of the outer layer of the shell, 

 or from having been abraded before the shell was embedded in sediment. Shell rather tliick 

 and formed of several layers or lamellae within a thin outer layer. The outer layer usually 

 adheres to the matrix, and the inner layers have been so replaced by calcareous matter that 

 the shell appears to be solid. In a few examples the individual layers are preserved, and the 

 oblique lamellae, encircHng the anterior portions of the valves, form imbricating lines where 

 the outer layer has been removed. Billings [lS61c, p. 947] described the shell as breaking 

 with a granular fracture, and when weathered as having a tendency to fibrous exfoliation, 

 which is true for the specimens in which the original structure is lost. 



The area of the ventral valve is usually lower than the greatest elevation of the surface 

 (PI. LIV, fig. la), but it may be as liigh as any portion of it (fig. Ic); it projects backward at an 

 angle varying from 45° to 70°. Billings states 45° to 60° from material he studied. The area 

 is divided midway by a narrow pseudo-pedicle furrow (p, fig. If). The area of the dorsal 

 valve is well defined in the cast (fig. Ig) and much like that of 0. crassa (Hall) (fig. 2k) and 0. 

 atlantica Walcott (Pl.LV,fig. 1 1). The two ridges on the cast that converge toward the posterior 

 margin are broader than in any other species except OJ lindstromi Walcott (PI. LV, fig. 4a). 



