PEOTOZOA I FORAMINIFERA. 49 



fisli order. In the true nautiloid shell the convolutions of the 

 spiral lie in a single plane, as in Rotalina, and the shell is said 

 to be 'equilateral.' In other cases, however, the spiral passes 

 obliquely round a central axis, and the shell becomes conical or 

 turreted, when it is said to be 'inequilateral' or 'trochoid.' 

 In other forms, such as Nummulites and Orbitolites, the struc- 

 ture of the shell, though regular, is much more complicated. 

 Besides these symmetrical forms, there exist others in which 

 the arrangement of the segments is very irregular, as is seen 

 in Globigerina, Acervulina, &c. {fig. 66). 



Besides the true pseudopodial foramina with which the 

 walls of the test in most of the Foraminifera are pierced, there 

 exists in some forms an additional system of complicated 

 branching and anastomosing tubes, which are distributed 

 between the laminae of the shell, and establish a communica- 

 tion between its external and internal surfaces. 



CLASSIFICATION OF FORAMINIFERA.^ The classification of the 

 Foraminifera has hitherto proved a matter of extreme difficulty, 

 and probably none of the arrangements as yet proposed can 

 be considered as more than provisional. The following is the 

 classification adopted by Dr, Carpenter, who is one of the 

 greatest living authorities upon the group. 



ORDER BETICULOSA. ( = FORAMINIFERA.) Rhizopods show- 

 ing no differentiation, or a very imperfect one, into ectosarc and 

 endosarc ; no nucleus or contractile vesicle ; pseudopodia fila- 

 mentous, minutely subdivided, and inosculating freely to form a 

 net-work. 



Section 1, Imperforata. Envelope membranous or calcar 

 reous, the walls not perforated by apertures for the pseudo- 

 podia, which are emitted solely from the single or multiple 

 aperture of the shell. 



Families, 1. Gromida. Test membranous. 



2. Miliolida, Test porcellanous, 



3. Lituolida. Test arenaceous, 



Section 2. Perforata, Envelope calcareous (hyaline or vi- 

 treous) or rarely arenaceous, its walls traversed by numerous 

 foramina for the emission of pseudopodia. 



The following classifications by D'Orbigny and Schultze are 

 founded merely upon the form of the shell, and, as such, are 

 purely arbitrary. Of the two Schultze' s arrangement is pro- 

 bably the more satisfactory. 



VOL. i. a 



