56 MANUAL or ZOOLOGY 



loid " shell, so common amongst the Foraminifera (fig. 4, d). 

 This type of shell is so closely similar to the shape of the Pearly 

 Nautilus, that the older naturalists were long in the habit of 

 classing these forms along with the Cephalopoda, or Cuttle- 

 fish 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 (fig. 5) and Orbitolites, the 

 structure of the shell, though regular, is much more compli- 

 cated. 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. 4, f). 



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 communication between its 

 external and internal surfaces. 



■ Classification ok For.4minifera. — The classification of 

 the Foraminifera has hitherto proved a matter of extreme diffi- 

 culty, 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 Reticulosa. ( = Foraminifera.) — Rhizopods show- 

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

 endosarc; no nucleus or contractile vesicle; pseudopodia filament- 

 ous, minutely subdivided, and inosculating freely to form a net- 

 work. 



Section i. Jmperforata. — Envelope membranous or calca- 

 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, i. Gromida. Test membranous. 



2. Miliolida. Test porcellanous. 



3. Lituolida. Test arenaceous. 



Section 2. Perforata. — Envelope calcareous (hyaline or vit- 

 reous) 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 



