72 UPPER CRETACEOUS-LOWER TERTIARY FORAMINIFERA 



because of its compressed last chambers and partially developed keel. Thus it is 

 considered here separately, in spite of the fact that it is represented by only one 

 species, until further study can reveal its true position. 



Globotrnncana is distinguished from Abathomphalus Bolli, Loeblich & Tappan 1957 

 by its interiomarginal, umbilical aperture, which is extraumbilical in Abathomphalus ; 

 by its generally large umbilicus, which is very much reduced in the latter genus ; 

 by its complex tegilla which is single in Abathomphalus and by the fact that its 

 accessory apertures are both infra- and intralaminal, not only infralaminal as in 

 A bathomphalus. 



Globotruncana differs from Praeglobotruncana Bermudez 1952 in its umbilical 

 aperture, umbilical cover-plate, and accessory apertures. It differs from both 

 Globorotalia Cushman 1927 and Hedbergella Bronnimann & Brown 1958 in the above 

 mentioned characters, as well as in the constant presence of a single or double keel. 



Remarks. The confusion surrounding most Globotruncana species has led the 

 author to split the present forms as much as their morphology and stratigraphical 

 ranges would allow. No splitting on the basis of minor morphological characters or 

 of rare specimens has been attempted. This has helped to clarify the nature of each of 

 the described forms, although further study (serial thin-sectioning and statistical 

 analysis) may favour the merging of some of these morphologically similar forms. 



The present study has shown that the characters of specific value within the genus 

 Globotruncana are as follows : 



1. The shape of the test (biconvex, planoconvex, concavoconvex, sprioconvex 



or parallel-sided) , which is a function of the relative shapes of both the dorsal 

 and the ventral sides. Variation within the range of each shape has been 

 observed, and is not of any taxonomic importance. 



2. The character of the keel, whether single or double, or transitional from one to 



the other ; and in the double-keeled forms the position of the two keels 

 relative to each other and to the rest of the test (parallel or divergent, 

 closely- or widely-spaced, equally- or unequally-developed, marginally 

 situated or shifted either to the dorsal or the ventral side), which affect the 

 size, shape and position of the peripheral band. 



3. The shape of the chambers on both the dorsal and ventral sides, the number of 



chambers in the test and in the last whorl, as well as the arrangement of the 

 chambers. This affects the general shape of the test, the shape of its 

 equatorial periphery (rounded, subrounded, polygonal, entire or lobate) ; 

 the character of the sutures on both sides of test (straight or curved, raised 

 or depressed), and the shape of the umbilicus. However, it should be 

 noted that, other things being equal, variation in any of these characters 

 separately is not of any taxonomic value. 



4. Character of the surface, whether smooth or rough, but not degree of rugosity. 



