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insriTED STATES NATIONAL AIUSEUM BULLETIN 216 



species are restricted to a single zone, fifteen to two zones, 

 ten to three zones. Only four species have a longer 

 range. Several groups of genetically closely related 

 species and subspecies can be distinguished. The as- 

 sumption of such genetic relationships is based on oc- 

 cmrences of morphologically transitional forms. To- 

 gether with the evolutionary trends it is also of interest 

 to foUow the ratios of the direction of coiling. It will be 

 shown in the foUowing section that such ratios may be 

 an indication of the stratigraphic position of a fauna 

 and help to verify the genetic relation between some 

 species and subspecies. 



The dominant suite of related species begins in the 

 Globorotalia trinidadensis zone with Globorotalia trinida- 

 densis BoUi, new species (text-fig. 12). Based on inter- 

 mediate forms it may be assumed that Globorotalia 

 pseudobulloides (Plummer) which also appears in this 



zone, is closely related to G. trinidadensis. Common an- 

 cestors might be found in beds equivalent in age to 

 those of the underlying Rzehakina epigorm zonule. In 

 the Globorotalia uncinata zone we find the zonal marker 

 developing from G. pseudobulloides (for a transitional 

 form, see pi. 17, figs. 16-18). G. uncinata BoUi, new 

 species, is regarded as the ancestor of G. angulata 

 (White). G. quadrata (White) is considered a separate 

 branch developing from G. trinidadensis. At the base of 

 the G. fusilla pusiUa zone, G. angulata apparently leads 

 through transitional forms to the long ranging G. aequa 

 Cushman and Renz. Before the extinction of G. aequa 

 at the end of the G. rex zone the two closely related 

 G. rex Martin and G. formosa gracilis BoUi, new spec- 

 ies, new subspecies, branch off. These two forms lead 

 in the foUowing zone to G. aragonensis jNuttaU and 

 G. formosa formosa BoUi, new species, new subspecies. 



Figure 12. — Tentative evolution of Globigerina and Globorotalia species in the Paleocene - lower Eocene, 

 Lizard Springs formation of Trinidad, B.W.I. 



