COMPOSITE FORMS OF FO R AMINIFE R A. 



439 



until it becomes a spiral (Fig. 205). The character of this spiral 

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

 ment of the successively formed chambers ; for sometimes it 

 opens out very rapidly, every whorl being considerably broader 

 than that which it surrounds, in consequence of the great excess 

 of the size of each segment over that of its predecessor, as is ge- 

 nerally the case in Cristellaria ; whilst in other instances there is 

 so little difference between the successive segments, after the 

 spiral has made two or three turns, that the breadth of each 

 whorl scarcely exceeds that of its predecessor, as is well seen in 

 Faujasina (Fig. 209), as also in Nummulite (Chap. XIX). An 

 intermediate condition is presented by Rosalina (Fig. 205), which 



FIG. 205. 



Rosalina ornata, with its pseudopodia extended. 



may be taken as an example of a very large group of Foramini- 

 fera, composed of those whose plan of growth is helical or spiral, 

 and ranged by M. D'Orbigny under the designation Helicostegues. 

 In this genus, as in a large proportion of its congeners, we find 

 the shell perforated with numerous apertures, through which 

 pseudopodia can be extended from any of the segments that are 

 not enclosed by others, as well as from the mouth or aperture of 

 the outer segment ; and when this is the case, it does not appear 

 that the sarcode body is so often extended over the exterior of 

 the shell, as it is when the shell has no perforations for the put- 

 ting forth of these extensions. There are generally indications, 

 however, in the structure of the shell itself, new layers being 

 often formed over the innermost whorls of the spiral, or partial 



