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PROCEEDINGS OF THE NATIONAL MUSEUM 



VOL. 86 



This form, which is of medium size, thin and compressed, and 

 completely involute, is described by Thalmann as having a diameter 

 of 3 to 4 mm; thickness 0.2 to 0.3 mm, and 31/2 ^'horls with 28 to 32 

 chambers in the last whorl. This, according to Thalmann, is for 

 the microspheric form, though from his figure this is not certain. 

 The surface is smooth in all specimens examined. 



Sections made from material collected by Thalmann, near his type 

 locality (various outcrops along the Tuxpam River between Cobos 

 and Tuxpam) show 3 to 314 whorls, with 20 to 24 chambers in the 

 final whorl. The coiling is somewhat irregular; the sutures thin, a 

 thick outer wall giving a thickened, rounded periphery; the chambers 

 are numerous, long, and narrow. 



Specimens from Biche Quarry, Nariva District, Trinidad, British 

 West Indies (the Guaracara limestone), in the collection of T. F. 

 Grimsdale, are considered to belong to this species. There seems to 

 be no doubt that Operculinoides tuxpanicus Vaughan and Cole is a 

 synonym of Operculina tuxpanensis Thalmann, though Thalmann un- 

 doubtedly had much better material than Vaughan. Various meas- 

 urements are given below for comparison : 



> Microspheric. 



Plesiotypes.—U.S.'NM. no. 497838. 

 Topotypes ( .?) .— U.S.N.M. no. 497837. 

 Occurrence. — Tuxpam formation, lower Miocene. 



OPERCULINOIDES MUIRI. new species 



Plate 14, Figure 4 ; Plate 20, Figube 1 ; Plate 22, Figukb 1 



Test small to medium in size, completely involute, lenticular and 

 rather close-coiled, with a fairly well developed, rounded keel of 

 clear shell material. Diameter, up to 3.0 mm (average for 10 spec- 

 imens, 2.6 mm) ; thickness, 0.7 to 0.9 mm. 



Median sections show regular, rather close coiling, with 4 to 4% 

 whorls, with 20 to 24 chambers in the final Avhorl. The sutures are 

 slightly oblique, curving rather strongly as they approach the 

 periphery. In transverse section the rather inflated lenticular form 



