STUDIES m FORAMTNIFERA 



55 



rugoglobigerinid forms. Like Qlohotruncana helvetica, 

 it is a short-lived offshoot from a Rugoglobigerina 

 species. The Maestrichtian Trinitella scotti Bronni- 

 mann {= Rugoglobigerina) with its compressed end 

 chambers is further proof of the repeated and seemingly 

 independent attempts of the Turonian-Maestrichtian 

 Rugoglobigerinae to develop one or two peripheral keels. 

 This brief outline of the probable phylogenetic pattern 

 demonstrates the close relationship between the genera 

 Rugoglobigerina and Globotruncana and at the same 

 time throws light on the artificial division into two 

 genera of planktonic Foraminifera that are genetically 

 closely related. The identical pattern of coUing is 

 further proof of such relationship. All species of both 

 groups maintain a strong preference for dextral coiling 

 from the Turonian to their contemporaneous extinction 

 in the Maestrichtian. An earlier evolutionary stage 

 with random coiling such as is found in certain Oligo- 

 Miocene Globorotalia species (BoUi, 1951) or in Rotali- 

 pora may be expected in Cenomanian ancestors. 



Acknowledgments 



The writer wishes to thank Mr. J. B. Saunders, 

 Palaeontologist of Trinidad OU Company, Ltd., and 

 Dr. H. G. Kugler, Consulting Geologist to Central 

 Mining Investment Corporation, for reading and dis- 

 cussing the manuscript. He is indebted to Dr. A. R. 

 Loeblich, Jr., U. S. National Museum, Washington, for 

 the support given towards the publication. The in- 

 formation contained in this paper is published by the 

 kind permission oi Trinidad Oil Company, Ltd., and 

 Trinidad Petrolum Development, Ltd. 



Illustrations for the present paper were prepared 

 under a grant to Dr. Alfred R. Loeblich, Jr., for plank- 

 tonic foraminiferal studies, administered by the Smith- 

 sonian Institution, for which funds were supplied by 

 the California Research Corporation, the Carter OU 

 Company, the Gulf Oil Corporation, and the Humble 

 Oil and Refining Company. These illustrations are 

 camera lucida drawings, prepared by Lawrence and 

 Patricia Isham, scientific illustrators, United States 

 National Museum. 



Systematic Descriptions 



Family Globorotaliidae Cushman, 1927 



Genus Praeglobotrimcana Bermudez, 1952 



Praeglobotruncana cf. delrioensis (Plummer) 



Plate 12, figures 4a-c 



Globorotalia delrioensis Plummer, Univ. Texas Bull. 3101, p. 

 199, pi. 13, figs. 2a-c, 1931. 



Shape of test: low trochospiral, biconvex; equatorial 

 periphery slightly lobate, no distinct keel. Wall: cal- 

 careous, perforate, surface smooth. Chambers: mod- 

 erately compressed; about 12, arranged in 2K to 3 

 whorls; the 5 chambers of the last whorl increase 

 rapidly in size; early whorls small by comparison. 

 Sutures: spiral side curved, depressed; umbilical side 

 nearly radial, depressed. UmbUicus: details obscure; 

 part appears to be covered by extensions of chambers. 

 Aperture: An interiomarginal, extraimibiUcal-umbilical 

 sUt. Coiling: Random; of the 8 specimens counted, 5 

 coiled dextrally. 



Dimensions of figured hypotype: Diameter 0.32 mm. ; 

 thickness 0.15 mm. 



Range: Globigerina washitensis zone and Rotalipora 

 appenninica appenninica zone, Gautier formation. 



Type and occurrence: Figured specimen (USNM 

 P4793) from Trinidad Leaseholds well Marac 1, Trini- 

 dad (coordinates N: 152141 links, E:424447 links), 

 sample at 9,773 feet (TLL 177171). 



Praeglobotruncana coarctata Belli, new species 

 Plate 12, figures 2a-3c 

 Shape of test: very low trochospiral, biconvex; 



equatorial periphery lobate; a faint keel is often ob- 

 served in last chambers; it may be ornamented with 

 minute spines on peripheral edge. Wall: calcareous, 

 perforate, siu-face smooth. Chambers: strongly com- 

 pressed; 10-12, arranged in 2 whorls; the 5-6 chambers 

 of the last whorl increase rapidly in size; the early 

 whorl minute by comparison. Sutures: spiral side 

 radial or slightly curved, depressed; umbUical side 

 radial, depressed. Umbihcus: each chamber of last 

 whorl extends towards the center, leaving only a small 

 portion open. Aperture: a low arched, interiomarginal, 

 extramnbiUcal-iunbihcal slit. Coiling: predominantly 

 dextral; of 37 specimens counted, only 4 coiled sinis- 

 trally. 



Dimensions of holotjrpe: diameter 0.4 mm.; thickness 

 0.16 mm. 



Range: Globotruncana stuarti zone, Naparima HiU 

 formation. 



Types and occurrence: Usine Ste. Madeleine 

 Quarry, southeast end of Naparima HUl, San Fernando, 

 Trinidad (coordinates N:235800 links, E:364000 hnks). 

 Holotype (USNM P4794) from sample Bt. 37 (TLL 

 151935); figured paratype (USNM P4795) from sample 

 Bt. 46 (TLL 151943). 



Remarks: Praeglobotruncana coarctata, new species, 

 difiFers from Praeglobotruncana citae (BoUi) in having 

 an almost flat spiral side instead of a convex one and in 

 having usually five chambers in the last whorl instead 

 of four. 



