110 



THE FORAMINIFERA 





great that we are led to doubt whether tenuissima is really allied 

 to them. On the other hand, the resem- 

 blance of the inner chambers of tenuissima to 

 Ophthalmidium,a,mGmber of the Hauerinidae, 

 suggests that it may be derived from this 

 family, and have acquired the cyclical mode 

 of growth independently. The acceptance 

 of this view is perhaps rendered easier by 

 the existence of another group, the Oper- 

 culino-Cydodypeus series, in the higher mem- 

 bers of which the annular mode of growth 

 is likewise attained (see p. 128). It seems 

 at any rate worth while to entertain the 

 possibility of this explanation, before ac- 



. J , . , 



cepting a conclusion so damaging to a body 



of evidence which may be found, if duly considered, to furnish 



the clue to many complicated problems of relationship. 1 



FIG. 41. 



Ophthalmidium tumiduluin, 

 ady. "Challenger" Report, 

 . XII. Fig. 6. 



Family Alveolinidae. The genus Alveolina which represents 

 this family contains a number of recent and fossil forms which 

 appear to branch off from the Miliolid stock in the neighbourhood 



Tests of (a) Alveolina boscii, Def., x about 17 ; and (b) 

 A. melo, F. and M., x about 22. 



of the genus OrUculina. They are char- 

 acterised by elongation of the chambers 

 in a plane at right angles to that in 

 which they are developed to form the 

 disc -shaped tests of Orbitolites that is, 



in the direction of the axis of the spire. The result is the 

 formation of a series of oblate, spherical, ovoid (Fig. 42, b), fusiform, 

 and cylindrical (Fig. 42, a) tests, each chamber extending beyond 

 its predecessors laterally to a greater or less extent, and thus 

 increasing the axial length of the test. The chambers are short 

 in the direction of the plane of the spire, and subdivided into 

 chamberlets by vertical septa lying parallel with that plane. In 



1 It would perhaps be premature, while we are not yet acquainted with the two 

 forms of tenuissima, to alter its systematic position, but should this view of its re- 

 lationship be confirmed, it must be separated as a distinct genus to which the name 

 Cydqphthalmidinm might be given. 



