74 UPPER CRETACEOUS-LOWER TERTIARY FORAMINIFERA 



& Brown 1956 and Cita 1963). However, the fact that the present study is mainly 

 concerned with the Maestrichtian Globotruncana, makes it difficult to go into detail, 

 although the phylogenetic development of each of the species discussed here, is 

 dealt with in the remarks on each species, and the various lineages suggested in the 

 present study are summarized in Text-fig. 10. The extension of such lineages 

 downwards in older strata can only be substantiated by the study of continuous 

 sections throughout the Upper Cretaceous. Nevertheless, the main evolutionary 

 tendencies observed in the various Globotruncana species discussed in the present 

 work can be briefly summarized as follows : 



(a) A tendency to reduce the ventral keel. 



(b) A tendency to reduce the size of test. 



(c) A tendency to increase the surface rugosity. 



Again, comparison with the known Globotruncana species in the Turonian, Coni- 

 acian, Santonian and Campanian, shows that : 



1. The tendencies towards reduction of the ventral keel in double-keeled Globo- 



truncana, and towards increase in surface rugosity exist throughout the 

 Upper Cretaceous. 



2. A tendency towards the gradual increase in the size of test is clearly documen- 



ted ; it reaches its maximum in the Lower Maestrichtian and is then 

 reversed towards the Upper Maestrichtian. 



3. A tendency to increase the height of coiling in spiroconvex forms is observed 



from the Turonian throughout the Maestrichtian, and manifests itself in the 

 flooding of Maestrichtian strata with representatives of the G. contusa 

 group, G. conica, G. esnehensis, G. sharawnaensis, G. orientalis and G. 

 fareedi. 



4. A tendency, upwards in the section, towards the modification of the shape of 



the chambers in the last one or two whorls from globular to ovoid, lenticular, 

 petaloid, crescentic, trapezoidal, rectangular or even polygonal, although 

 some of the last representatives still maintain the globular shape of the 

 chambers. This modification of the chamber shape in the last one or two 

 whorls affects the general shape of the test and also the shape and size of 

 the umbilicus. 



5. There is a general increase in the number of individuals of each species and in 



the number of species and subspecies between the Turonian and the Maes- 

 trichtian. This is accelerated in the uppermost Cretaceous, and results in 

 the younger species having a much shorter range than the older ones. 



These tendencies, in general, agree well with previous observations by other 

 authors, especially Bronnimann & Brown (1956), who also noted tendencies towards 

 the "refinement of shell material " and towards an increase in the size of the aper- 

 tural flaps at stratigraphically higher levels. Although exceptions have been noted 

 to the above-mentioned trends, their existence is in no way invalidated. 



