PROTOZOA : FORAMINIFERA. 53 



single " primordial segment " in every respect identical with 

 the permanent condition of a simple species. They commence 

 their existence, therefore, as Monothalamia, and are converted 

 into Polythalamia merely by a process of " vegetative " or " irre- 

 lative repetition." As their development proceeds, the primitive 

 mass of sarcode, or "primordial segment," throws out fresh 

 segments in the form of buds according to a determinate law ; 

 and it is upon the direction in which these segments are 

 evolved that the ultimate form of the shell depends. The 

 more important variations in this respect are as follows : If 

 the additional segments are added to the primordial chamber 

 in a linear series, so as to form a straight or slightly curved 

 line, we obtain respectively a Nodosaria (fig. 4, d, e) or a Den- 

 talina. When the new chambers are added in a spiral direction, 

 each being a little larger than the one which preceded it, and 

 the coils of the spiral lying in one plane, then we get the " nauti- 

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

 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 OF FORAMINIFERA. 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, info ectosarc and 



