310 PROCEEDINGS OF THE NATIONAL MUSEUM vol.83 



with an observed maximum of 7.0 mm; thickness, up to 1.0 mm, with 

 an average of between 0,8 and 0.9 mm. 



Sections show the coiling to be regular, mature specimens showing 

 5 to 6 whorls with 37 or 38 chambers in the final one. The septa 

 are thin and slightly sigmoid in shape, recurving sharply toward tha 

 periphery. Chambers numerous, rather long in proportion to their 

 width. Transverse sections show that there is a tendency for the 

 outer walls of succeeding coils to become closely appressed, without, 

 however, becoming fused. 



Pleswfypes.—U.S.l^M. nos. 497829 and 497830. 



Other specimens.— U.S.^M. nos. 497831 and 497832. 



Occurrence. — Common in the Tantoyuca formation, Jackson Eocene. 



OPERCULINOIDES NUMMULITIFORMIS (L. Rutten) 



Plate 17, Figure 5 ; Plate 21, Fiquke 1 



1928. Operculina nummuUtifonnis L. Rutten, Proc. Sect. S?ei. Kou. Akad. 



Wetensch., Amsterdam, vol. 31, no. 9, p. 941, figs. 1-12. 

 1932. Operculina nummulitiformis L. Rutten, M. G. Rutten and VEaiiiuNT, Proc. 



Sect. Sci. Kon. Akad. Wetensch., Amsterdam, vol. 35, no. 2, p. 239, 



pi. 1, figs. 7, 10; pi. 2, fig. 1. (Full synonymy given in this account.) 

 1937. OpercuUnella nmnmuVitiformis (L. Rutten) Vaughan, in Sheppard's 



"The Geology of South-Western Ecuador," pp. 159-160, figs. 116 (1-3). 



Mexican specimens identified as this species are described as 

 follows : 



Test of medium size, very compressed, completely involute. The 

 sutures are strongly beaded and slightly raised, the test being a 

 little thicker in proportion to diameter than in 0. prenummuUti- 

 formis. Diameter, average 8.0 mm; thickness, average 0.5 mm. 



Sections show numerous long narrow chambers, the test showing 

 31/2 to 41/2 whorls, with 28 to 33 chambers in the final whorl. The 

 septa' are rather irregular as shown in Rutten's original figures of 

 O. nummulitiformis. The Mexican specimens differ from L. Rutten's 

 original description in that they are slightly thicker (0.5 nnn average 

 compared with 0.35-0.45 mm) and from Rutten and Vermunt's de- 

 scription in having beaded sutures in place of a smooth surface. This 

 latter difference may be due, perhaps, to state of preservation and to 

 local variation, as the degree of beading is variable in specimens 

 examined. In other respects the species are remarkably similar, and 

 Rutten and Vermunt's figure (1932, pi. 2, fig. 1) leaves little doubt 

 in my mind that the Mexican specimens should be referred to O. nu7n- 

 mulitiformis. It seems doubtful Avhether Vaughan's Ecuadorian 

 material should be referred to this species, since the specimens fig- 

 ured show a more rapidly opening spiral with fewer whorls and 

 fewer chambers in the final whorl in proportion to the size of the 



