STUDIES IN FORAMINIFERA 



11 



Biconvex. Both spiral and umbilical sides convex, 



or more or less inflated, e. g., Globorotalia tumida 



(Brady). 

 Spiroconvex. Spiral side convex, xmabilical side 



flattened to concave, e. g., Olohotruneana contusa 



(Cushman). 

 Umbilico-convex. Umbilical side convex, spiral 



side flattened to concave, e. g., Globorotalia trun- 



catulinoides (d'Orbigny). 

 Enrolled biserial: This is a modification of the 

 planispiral development in which biserially alter- 

 nating chambers are enrolled. Characteristic of 

 the nonplanktonic family Cassidulinidae, this type 

 of coiling is also found in Cassigerinella, a genus of 

 the family Hantkeninidae. 



Nonumbilicate 



Lacking an mnbilicus (text-fig. 2). 



Streptospiral: In the planktonic Foraminifera this 

 may be a later modification of the trochospiral coil- 

 ing, in which the plane of coiling continually chang- 

 es, as in the coiling of a ball of string. As the plane 

 of coiling changes, the axis of coiling changes, hence 

 no umbilicus is formed at the terminus of the axis 

 of coiling, e. g., Pulleniatina. 



Globular: A globular test may be formed, by a com- 

 pletely enveloping final chamber as in Orbulina; or 

 by the development of a many chambered test, 

 with rapid increase in chamber size and commonly 

 considerably embracing later chambers as in Glo- 

 bigerinatheka and Globigerinatella. 



Structures of Test 



Primary Chambers 



The chambers whose pattern of development deter- 

 mine the test shape and form (text-fig. 3). 



Angular conical: Inflated chambers with angular 

 margins and a conical form as in Globorotalia 

 truncatulinoides (d 'Orbigny ) . 



Angular rhomboid: Chambers with rhombic sec- 

 tion and sharply angled as in Rotalipora brotzeni 

 (Sigal). 



Angular truncate: Chambers inflated but with 

 truncate margins, angular and commonly keeled, 

 e. g., Globolruncana area (Cushman). 



Ovate: Chambers moderately inflated and ovate in 

 section, e. g., Rotalipora roberti (Gandolfi). 



Hemispherical : Chambers inflated at one side, flat- 

 tened on the opposite side, and thus hemispherical 

 as in Globolruncana helvetica Bolli. 



Spherical 



Ovate 



Hemispherical Clavate 



Radial Elongate 



Tubulospinate Angular Conical Angular Rhomboid Angular Truncate 



Figure 3. — Chamber shapes in planktonic Foraminifera. 



