ii 4 UPPER CRETACEOUS-LOWER TERTIARY FORAMINIFERA 



Globotruncana gansseri dicarinata Pessagno 



(PI. 5, figs. 4a-d) 



i960 Globotruncana (Rugotruncana) gansseri dicarinata Pessagno : 103, pi. 2, figs. 9-1 1 ; 

 pi. 3, figs. 1-3 ; pi. 5, fig. 2. 



Description. Test large, planoconvex, umbilico-convex, coiled in a very low 

 trochospire ; dorsal side flat, ventral side strongly protruding, equatorial periphery 

 roughly ovoid or rather quadrate, distinctly lobate, with two well-developed, 

 thickened, heavily beaded, widely spaced keels, reduced to a single keel on the last 

 chamber ; axial periphery truncate in the early part, subangular in the later ; 

 chambers on the dorsal side are not all clear because of the surface rugosity, but 

 appear to be 16 in number, arranged in 3 dextrally coiled whorls ; the initial chambers 

 are extremely small, indistinct, almost masked by the surface rugosity, increase very 

 slowly in size and are followed by globular to crescentic, weakly inflated chambers 

 which also increase slowly in size ; the last whorl is composed of 5 large, crescentic 

 chambers which increase rapidly in size ; on the ventral side the chambers are 5 

 large, angular conical and strongly protruding ; sutures on the dorsal side curved, 

 raised and heavily beaded ; on the ventral side they are straight, radial and depres- 

 sed ; umbilicus roughly pentagonal in outline, wide, deep, surrounded by a raised, 

 beaded, umbilical flange in the early part, which fades out gradually towards the last 

 chamber, and is covered by complex tegilla of which remnants are still preserved; 

 primary apertures interiomarginal, umbilical ; tegilla with accessory apertures only 

 poorly preserved ; wall calcareous, perforate except for the imperforate keels, 

 peripheral band and tegilla ; surface rough, heavily papillose, nodose, or spinose in 

 the early part with the rugosity decreasing gradually towards the last chamber. 



Dimensions of described specimen. 

 Maximum diameter = 0-50 mm. 



Minimum diameter = 0-35 mm. 



Thickness = 0-26 mm. 



Remarks. Pessagno (i960) stated that this subspecies differs from G. gansseri 

 gansseri Bolli in having a distinct double keel and well-developed rugosities in the 

 early stages. He also added that it is intermediate between Rugoglobigerina rugosa 

 subrugosa Gandolfi and G. gansseri gansseri Bolli and that it has evolved from the 

 former by dorsal flattening and the migration of the double-keel band to the dorsal 

 periphery. However, this subspecies appears to be more closely related to G. 

 gagnebini Tilev from which it differs only by the well developed surface rugosity. 

 Thus its assignment to G. gansseri may seem doubtful, as the last mentioned species 

 is characterized by its entirely single keel. However, the fact that Bronnimann & 

 Brown (1956) and Pessagno (i960) observed a double-keeled rugoglobigerine 

 nepionic stage in thin sections of G. gansseri gansseri, may support Pessango's 

 hypothesis that the latter subspecies has evolved from G. gansseri dicarinata by the 

 gradual reduction of the ventral keel. The specimen here figured as G. gansseri 

 dicarinata Pessagno conforms well with the holotype, while the paratype figured 

 by Pessagno (i960) lacks the typically crescentic shape of the chambers on the 



