FORAMINIFERA AND RADIOLARIA. 



67 



successive segments are added in a spiral direction, the character of the 

 spire will depend in great degree upon the enlargement or non-enlargement 

 of the successively-formed chambers; for sometimes it opens out very 

 rapidly, every whorl being considerably broader than that which it sur- 

 rounds, in consequence of the great excess of the size of each segment 

 over that of its predecessor, as in Peneroplis ; but more commonly there 

 is so little difference between the successive segments, after the spire has 

 made two or three turns, that the breadth of each whorl scarcely exceeds 

 that of its predecessor, as is well seen in the section of the Rotalia rep- 

 resented in Fig. 330. An intermediate condition is presented by such a 

 Rotalia as is shown in Fig. 314, which may be taken as a characteristic 

 type of a very large and important group of Foraminifera, whose general 

 features will be presently described. Again, a spiral may be either ' nau- 



FIG. 314. 



Rotalia ornata, with its pseudopodia extended. 



tiloid ' or ' turbinoid': the former designation being applied to that form 

 in which the successive convolutions all lie in one plane (as they do in 

 the Nautilus), so that the shell is ( equilateral ' or similar on its two sides; 

 whilst the latter is used to mark that form in which the spire passes 

 obliquely round an axis, so that the shell becomes ' inequilateral/ hav- 

 ing a more or less conical form, like that of a Snail or a Periwinkle, the 

 first-formed chamber being at the apex. Of the former we have charac- 

 teristic examples in Polystomelia (Plate xv., fig. 16) and Nonionina 

 (fig. 19); whilst of the latter we find a typical representation in Rotalia 

 Baccarii (fig. 18). Farther, we find among the shells whose increase 

 takes place upon the spiral plan, a very marked difference as to the de- 

 gree in which the earlier convolutions are invested and concealed by the 

 latter. In the great Rotaline group, whose characteristic form is a tur- 



