STUDIES IN FORAMINIFERA 



15 



Wall of Test 



Composition 



In the families under consideration, the wall is 

 composed wholly of calcium carbonate (calcite). 



Structure 



All these planktonic genera have a perforate radial 

 wall structure. 



Bronnimann and Brown (1956) stated that the gen- 

 era of the Globotrimcanidae (including some genera 

 here placed in the Globorotaliidae) have granular per- 

 forate walls, except for the surface ornamentation of 

 keels, pustules, etc., which are imperforate. Wood 

 (1949) had stated earlier that Globotruncana has a per- 

 forate radial wall structure, hence we have checked 

 the wall of the various planktonic genera here described, 

 and have foimd the wall of each to be perforate radial. 

 As noted by Bronnimann and Brown, and earlier by 

 Wood, the ornamentation of these genera, and that of 

 many of the other genera and famUies of Foraminifera, 

 may consist of apparently imperforate or very finely 

 perforate shell material that is nevertheless quite dis- 

 tinct from the type of material of the imperforate or 

 porceUanous Foraminifera. 



Because many workers have had difficulty in correctly 

 determining the wall structure of various Foraminifera, 

 and wrongly determine the shell of some to be granular, 

 we are here giving the method used in these determi- 

 nations. Wood (1949) gave an excellent siunmary of 

 the wall characters of many genera and species, but 

 his photographs of entire specimens of Foraminifera, 

 to demonstrate the typical appearance of the diiferent 

 types, have apparently mislead some later workers. 

 As was clearly stated by Wood in his text, however, 

 the wall structure may be quite obscure if entire shells 

 are examined in polarized light, especially if the walls 

 are relatively thick. In this case, either fragmented 

 specimens or thin sections must be used. The former 

 method, being usually the quicker, is as follows: A 

 clean specimen, free from extraneous filling if possible, 

 is placed on a glass sUde and gently crushed with pres- 

 sure of another glass slide above. A drop of oU, of 

 the index of refraction of calcite, is then added, the 

 cover glass replaced above, and a fragment is sought 

 which shows the wall in cross section. In this frag- 

 ment the radial or granular structure can be easily 

 determined in polarized light. Further details of the 

 appearance of the fragments of various types of wall 

 structure are given by Wood (1949). 



Surface 



The sm^ace ornamentation is here considered to be 

 of specific importance only. The following terms are 

 in common use in specific descriptions. 



Smooth: E. g., Candeina nitida d'Orbigny. 

 Cancellate: With a honeycomb-like surface, e. g., 



Glohigerina reticulata Stache. 

 Spinose : With very fine solid spines, generaUy elon- 

 gate, e. g., HastigerineUa rhumbleri Galloway. 



Hispid: Very fine, short, and hair-Hke "spines" as 

 in Globorotalia truncatulinoides (d'Orbigny). 



Rugose: Rough irregular ornamentation, which may 

 form ridges, e. g., Rugoglobigerina rugosa (Plum- 

 mer). 



Beaded: Small rounded elevations or "beads" which 

 commonly occur along the sutures and keels, but 

 may also occur on the chamber wall as in Globo- 

 truncana area (Cushman). 



Pitted: Small, generally rounded depressions in the 

 surface of the wall, e. g., Sphaeroidinella dehiscens 

 (Parker and Jones). 



Evolutionary Trends 



By a study of the geologic record in combination 

 with the ontogeny of the species, several evolutionary 

 trends may be noted. In general these trends are largely 

 related to the pelagic nature of the organism, tending 

 on the one hand to develop a lighter test (thus decreas- 

 ing the specific gravity and so enabling it to float) or to 

 develop a flattened or radial form (thus retarding its 

 sinking by offering increased area of resistance to the 

 water) and on the other hand a tendency to develop a 

 heavier adult test by the addition of more shell mate- 

 rial. These tendencies are undoubtedly the residt of 

 selective survival, but may be enumerated as follows: 



Replacement of a single primary aperture by many 

 smaller openings. This is accomplished in various 

 ways: 1. By the development of lateral relict supple- 

 mentary apertures, and in BiglobigerineUa in developing 

 paired apertures. 2. Development of multiple areal 

 supplementary apertures as in Cribrohantkenina. 3. 

 Development of sutural supplementary apertures on 

 the spiral side as in Globigerinoides and Truncorotaloides, 

 on the umbilical side as in Rotalipora, or on both spiral 

 and umbilical sides as in Candeina. 4. Development 

 of accessory intralaminal or infralaminal apertures as 

 in Globotruncana or Globigerinita. 



Obscuring or covering of the aperture. The simpler 

 forms have relatively uncomphcated and open aper- 

 tures, but later developments such as the following 

 may obscure the primary apertures: 1. Apertural lips 

 (e. g., Globorotalia) or umbilical "teeth" (e. g., Globo- 

 quadrina). 2. Chamber extensions, e. g., the flanges 

 of Sphaeroidinella and tegiUa of Globotruncana. 3. Ac- 

 cessory structures or bullae, e. g., Globigerinita. 4. 

 Enveloping final chambers, e. g., Orbulina. 5. Enfold- 

 ing of chambers by development of streptospiral coil- 

 ing, e. g., Pulleniatina. 



Tendency to develop a spherical test: 1. By means 

 of enveloping chambers, e. g., Orbulina. 2. By becom- 

 ing streptospiral in development, e. g., Pulleniatina. 

 3. By much inflated chambers in planispiral genera, 

 e. g., Hastigerina. 4. By becoming high spired in 

 trochospiral genera, e. g., Globigerinoides. 



Tendency to develop a radial form: 1. By develop- 

 ment of radial elongate chambers as in HastigerineUa, 

 Hantkenina aragonensis, and Rugoglobigerina scotti. 



