•UNITED STATES NATIONAL MUSETJM BULLETIN 215 



flanges; the general shape of the chanabers (compressed 

 or globular) ; and the surface of the wall (smooth or 

 spinose). Other features, such as the number and the 

 rate of increase in size of the chambers or the nature of 

 the sutures (oblique or horizontal, straight or curved) 

 are more variable, but can in certain cases be used for 

 the distinction of subspecies. 



Whereas the aperture is usually characteristic for 

 each species, there is a considerable diversity within 

 the whole Chiloguemhelina group. Extremely asym- 

 metrical apertures with a transparent flange occur in 

 Chiloguemhelina midwayensis (Cushman) and Chilo- 

 guembtlina martini (Pijpers) (pi. 21, figs. 1-3, 6, 14). 

 The asymmetrical shape and position of the apertm-e 

 is not due to lateral compression or distortion of the 

 test, but is a character which alternates regularly 

 within one specimen as a result of the biserial arrange- 

 ment of the chambers. On the other hand, Chiloguem- 

 helina wilcoxensis (Cushman and Ponton) and Chilo- 

 guemhelina trinitatensis (Cushman and Renz) have a 

 symmetrical, semicircular to crescentic aperture, similar 

 to that of many Cretaceous species of Heterohelix (pi. 21, 

 figs. 7, 10, 12). Another variant is Chiloguemhelina 

 paraUela, new species, where the aperture is high and 

 narrow, symmetrical in shape and situated in the center 

 of the apertural face (pi. 21, fig. 8). 



In ChUogvsmhelina midwayensis suhcylindrica, new 

 subspecies, C. paraUela, new species, and C. wilcoxensis 

 (Cushman and Ponton), the aperture of the last regular 

 chamber is occasionally covered by a small chamber of 

 irregular shape (pi. 21, figs. 3, 13; text-fig. 15, Nos. 

 36, 38, 56). The wall surface of this small chamber is 

 usually smoother than that of the previous chambers. 

 This featm-e resembles the terminal chamber of Zeauvi- 

 gerina and suggests a close relationship between this 

 genus and Chiloguemhelina. The tubular neck char- 

 acteristic of Zeauvigerina is, however, absent in Chilo- 

 guemhelina. 



It is interesting to note that Chiloguemhelina mid- 

 wayensis suhcylindrica, new subspecies, C. parallela, new 

 species, and C. wilcoxensis (Cushman and Ponton), 

 the only three species which have this end chamber, 

 seem to be the last stages of three different evolutionary 

 lines, as follows: 



midwayensis 

 suboylindilca 



sabtriangalarls 



This suggests that the end chamber in Chiloguemhelinxi 

 is a gerontic stage, which is developed shortly before the 

 extinction of an evolutionary line. 



Evolutionary Trends and Relationships to Other 

 Genera 



It is easy to recognize evolutionary trends in the 

 Paleocene-lower Eocene Chiloguemhelina species from 

 the Lizard Springs formation. The faunas are well 

 preserved and contain intermediate forms which radi- 

 cate the origin of the various species. On the other 

 hand, it has not been possible to trace definite evolu- 

 tionary trends in Chiloguemhelina within the Navet and 

 Cipero formations. 



The preceding discussion of morphological details 

 indicates that the genus Chiloguemhelina includes 

 species showing various apertural characteristics. Dis- 

 tinctive featiu"es, however, such as a symmetrical aper- 

 ture or a small terminal chamber, occur independently 

 in different evolutionary lines. Species showing various 

 types of apertures and shapes of the test are apparently 

 closely related and it seems therefore reasonable to 

 include them in one single genus. 



The main featm'es distinguishing Chiloguemhelina 

 from Heterohelix, are the absence of a coiled early stage, 

 the tendency to develop a twisted test, and the presence 

 of necklike extensions or flaps around the aperture. 

 The chamber arrangement is biserial as in Bolivina, and 

 some species of Chiloguemhelina and Bolivina are similar 

 in appearance. However, Chiloguemhelina has inflated 

 chambers, no ornamentation, but often a hispid wall 

 surface. The aperture is rarely so high and narrow as 

 in typical Bolivina. Fvtrther characteristics of Chilo- 

 guemhelina are the twisted test and the absence of an 

 ■ internal structure connecting the apertvu-es of successive 

 chambers. There is also a difference in the habitat of 

 the two genera. The frequency of Chiloguemhelina in 

 Glohigerina marls suggests a planktonic mode of life for 

 this genus, whereas Bolivina is generally regarded as 

 bottom-hving. 



The relationship between Chiloguemhelina and Zeau- 

 vigerina has been mentioned above. 



Stratigraphic Occurrence 



The stratigraphic range (see text-fig. 16) of Chilo- 

 guemhelina in Trinidad is from Paleocene to Oligocene. 

 This is in agreement with the observations of most 

 previous authors. None of the Chiloguemhelina species 

 described in this paper occur in the Upper Cretaceous 

 of Trinidad. The author has not systematically 

 checked upper Oligocene, Miocene, or Recent faunas 

 for the presence of Chiloguemhelina, and it is possible 

 that additional species will be found in these faunas. 



The variety of species of Chiloguemhelina reaches a 

 first climax aroimd the Paleocene-lower Eocene bound- 



