318 PROCEEDINGS OF THE NATIONAL INIUSEUM vol.86 



OPERCULINOIDES VICKSBURGENSIS Vaughan and Cole 



Plate 12, Figuke 6; Plate 18, Figitbe 2; Plate 19, Figures S, 9 



1936. Opercitlmoides vicksburgcnsis Vaughan and Coi-e, Proc. U. S. Nat. Miis., 

 vol. 83, p. 490, pi. 36 (Byram Marl, Vicksburg, Jliss.). 



Description of Mexican specimens is as follows: Test small to 

 medium size, compressed lenticular, completely involute, periphery 

 rather thick and rounded. The sutures as seen on the exterior are 

 clear lines of shell material, flush with the surface, and slightly sig- 

 moid in shape. The most important feature in the external appear- 

 ance is the presence of thin lines of clear shell material similar to 

 the subdivision into chamberlets shown by Hefet'osteginu; these dis- 

 appear on sectioning, however, or merely show as thin transparent 

 lines in the shell wall. This character was also seen to be well de- 

 veloped in specimens identified by the writer as O. vickshurgensis 

 from the Byram marl of Byram, Miss, (see pi. 19, figs. 8, 9). Diame- 

 ter, 2.0 to 3.0, average approximately '2.5 mm (compare 1.3 to 3.1 

 mm for O. vickshurgensis) . 



Sections show the presence of 3 to 31 1 whorls, with 18 to 24 cham- 

 bers in the last whorl. This compares closely with ?>Yi to 4 whorls, 

 with 18 to 26 chambers in the final whorl in O. vickshurgensis. The 

 septa are straight for two-thirds of their length, then recurve regu- 

 larly and rather abruptly toward the periphery. 



Plesiotypes.—JJ.S.'^M. no. 497863. 



Occurrence. — Alazan formation (Huasteca formation of Muir), 

 lower Oligocene. (Occurs generally in association with O. rnuiri.) 



Note. — There is clearly an error in the footnote given by Thiadena 

 (1937, p. 97), referring his Camerina sp. C to this species, since the 

 former, in my opinion, is either Plamdaria or Cristellaria (see Thia- 

 dens's pi. 15, fig. 4) . 



OPERCULINOIDES OLIVERI (Cushman) 



Plate 11, Figure 1; Pl.slte 1.5, Figire 3 



1925. OpercuUna olivcri Cushman, Bull. Ainer. Assoc. Petr. GooL, vol. 9, p. 298, 



pi. 6, figs. 1, 2 (Guayabal, Rio Moctezuma, Mexico). 

 1927. OpercuUna cushmani Cole, Bull. Amer. Pal., vol. 14, no. 51. p. 23, pi. 2, 

 fig. 14 (Guayabal tj^e locality, Guayabal, Mexico). 



Test large, involute, very thin, and complanate. From the ex- 

 terior the test is seen to consist of a rapidly opening spiral, the septa 

 showing as raised ribs, recurved strongly toward the periphery. 

 There is at times a tendency toward beading on the septa, this being 

 usually more strongly developed toward the center of the test. Di- 

 ameter, up to 8.0 mm, averaging 4.0 mm. 



Sections show the presence of 2 to 3 whorls, with 16 to 28 cham- 

 bers in the final whorl. This large range is due to the inclusion of 



