102 PROTOZOA. 



the' spiral all lie in one plane; but in other cases, as in 

 Eotalia (fig. 16), the shell becomes turreted or top-shaped, 

 in consequence of the coils of the spiral passing obliquely 

 round a central axis. 



Fig. 16. Rotalia Boueana. (D'Orbigny.) 



Ill a few types of the Foraminifera (e.g., in the Dactyl - 

 oporidce, fig. 19) the successive chambers of the multilocular 

 test have no direct communication with one another, and 

 simply cohere by their walls. In the majority of the com- 

 pound shells, the successive chambers are so produced, that 

 the septum between any two of thehi is formed solely by the 

 anterior wall of the older chamber, which thus constitutes 

 the posterior wall of the newer one (fig. 13, e). In the 

 highest types of the compound Foraminifera, however, each 

 segment is provided with its own proper wall of shell, each 

 segment, as it is produced, forming for itself a posterior 

 wall which applies itself to the anterior wall of the preceding 

 segment, so that each septum (" septal plane ") is composed 

 of two lamellae, as seen in fig. 1 7, A (Carpenter). Moreover, 

 " in the higher types of the hyaline or vitreous series we 

 frequently meet with an ' intermediate ' or ' supplemental ' 

 skeleton, formed by a secondary or exogenous deposit upon 

 the outer walls of the chambers, by which they receive a 

 great accession of strength. This deposit not only fills up 

 what would otherwise be superficial hollows at the junctions 

 of the chambers (fig. 17, A, d\ or (as in Polystomella) at the 

 umbilical depression, but often forms a layer of considerable 

 thickness over the whole surface, thus separating each whorl 

 from that which encloses it ; and it is sometimes prolonged 

 into outgrowths that give a very peculiar variety to the 

 ordinary contour, as in some varieties of Rotalia and Poly- 



