STUDIES IN FORAMINIFEEA 



possibility of a direct effect of quantity of light on the 

 Foraminifera. This has been demonstrated by Myers 

 (1943, p. 453) on benthonic species, and was suggested 

 as a possibility in the distribution of some planktonic 

 species by Wiseman and Ovey (1950, p. 63). Then- 

 sample 8 from the south Atlantic contained a fauna 

 typical of warmer water than did that of sample 2 from 

 the north Atlantic, although the actual water tempera- 

 tiu-e of sample 8 was lower than that of sample 2. 

 "From the positions of these two samples (number 2 

 is farther from the equator than is number 8) there is a 

 much closer relationship with latitude than with tem- 

 peratm^e, which suggests the possibility that the dis- 

 tribution of the northern and southern cold species 

 Olobigerina pachyderma and 6. dutertrei are at least 

 partially governed by the low illumination in these 

 latitudes ..." Thus the amount of Hght may also 

 be a factor in the distribution of species. 



Salinity and suspended sediments: According to 

 Ovey (1948, p. 6), for the existence of pelagic Fora- 

 minifera deep water is not necessary, but "it appears 

 only to be necessary to have water free from land- 

 derived pollution by river sediments. Globigerinidae 

 are often foimd in the Mediterranean, for example, in 

 association with relatively shallow water benthic 

 forms, but wherever found the water above has been 

 clear." 



This was substantiated by F. Parker (1954, p. 478) 

 in work on sediments of the Gulf of Mexico. She found 

 planktonic specimens to be much rarer in the region of 

 the Mississippi River delta than elsewhere. They do 

 not occur at all in sediments as shoal as in the rest of 

 the area, and she stated, "Their absence in the delta 

 region is probably due to the outflow of the Mississippi 

 River which causes water to flow out over the surface 

 for long distances." She considered (p. 472) that the 

 salinity was not affected much thereby at the shoalest 

 stations, and probably was not a controlling factor for 

 the faunal changes at either side of the delta region. 

 Quite possibly the large amoimt of sediment in the 

 water in this area of delta formation is the factor con- 

 trolling the planktonic population. 



Similar evidence of a control by suspended sediments 

 is found in fossil material. Although in general the 

 Cretaceous has an abundant planktonic fauna, there are 

 sediments which wholly lack them. An example is the 

 Cretaceous sequence of northern Alaska which con- 

 tains a fairly large total fauna of benthonic species 

 (approximately 200 species). Planktonic species are 

 absent, however, throughout the entire section ranging 

 from Neocomian through Senonian, except for one thin 

 horizon of Tiu-onian age which contains two or three 

 planktonic species. Tappan (1951, p. 4) stated: "The 

 Alaskan Cretaceous is thus equivalent in age to a portion 

 of the very fossiliferous Cretaceous sediments of the 

 GuK Coast, but the faunas have little in common other 

 than age. The Gulf Coast faima is extremely varied 

 with many pelagic forms and a great abundance of 

 calcareous and specialized types, but the Cretaceous of 



Alaska contains a dominantly arenaceous faima and 

 has almost no specialized forms." This difference was 

 explained as environmental, as "the Alaskan section 

 contains sands and clays but no hmestones, and the 

 clastic sediments are neither clean nor well-sorted, thus 

 suggesting rapid sedimentation and muddy waters." 

 This "graywacke" type of sediment is always very 

 poor in pelagic species, although they may occur in 

 contemporaneous sediments of differing Uthologic type. 



Morphology and Terminology 



In order to avoid repetition, the morphology of the 

 various planktonic genera is more fully discussed in the 

 section on systematics. However, a general discussion 

 is given here, with definitions of the terminology used. 



In the past there has been little agreement in the 

 descriptive terminology used in defining the genera of 

 Foraminifera. Brotzen (1942, p. 11) first used a more 

 exact terminology in defining apertures and their 

 position. He defined septal apertures and lateral 

 apertures. The former could be interiomarginal, exterio- 

 marginal, or areal in position, or there could be com- 

 posite apertures with one in each of two or more 

 positions. 



The lateral apertures could be either lateral, latero- 

 marginal, or sutural, according to Brotzen. This was 

 a considerable advance over the earlier statements 

 such as "at the base of the final chamber," but it did 

 not take into consideration the origin of the apertures 

 and their relative importance. Furthermore there are 

 various types of openings in the planktonic Foramini- 

 fera which do not fit well into Brotzen's classification. 



The types of coiUng have also been variously termed. 

 That of Globorotalia for example has been termed 

 trochoid by Cushman, rotaloid by GaUoway, turbo- 

 spiral by Brotzen and trochospiral by Glaessner. 



The two sides of the test in these asymmetrical forms 

 have also been variously named. Cushman (1948, 

 p. 16) referred to them as dorsal and ventral, the dorsal 

 side being that on which the chambers of all the whorls 

 are visible. Galloway defined ventral as "pertaining 

 to the inferior side, particularly the apertural side in 

 coiled forms; opposite the dorsal side." Dorsal was 

 stated (1933, p. 464) to be "pertaining to the back; 

 opposite to the ventral side." But in some genera of 

 planktonic Foraminifera the primary "ventral" aper- 

 ture is closed; there are apertures only on the "dorsal" 

 side, and none on the "ventral." 



Glaessner (1948, p. 69) defined the dorsal side in 

 high-spired forms as that with the apical surface of the 

 spire, and the base he considered to be the ventral side. 

 He added that in low-spired forms "the evolute side is 

 usually referred to as dorsal and the involute side 

 as ventral." 



However, some benthonic genera are attached by 

 the side showing all the whorls, which in life was there- 

 fore "ventral" or inferior, and the aperture may also 

 appear on this evolutely spiral side. Thus in these 



