FORAMINIFERA 



797 



(fig. 16), so that the composite shell has a conical form, the apex of 

 the cone being the original segment, and its base the last one formed. 

 The method of growth now described is common to a large number 

 of Foraminifera, chiefly belonging to the sub-family Nodosarince, ; 

 but even in that group we have every gradation between the recti- 

 lineal (fig. 16) and the spiral mode of growth (fig. 22) ; whilst in 

 the genus Peneroplis it is not at all uncommon for shells which com- 

 mence in a spiral to exchange this in a more advanced stage for the 

 rectilineal habit. When the 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 surrounds, in con- 

 sequence of the great excess of the size of each segment over that of 

 its predecessor, as in Peneroplis, fig. 606 ; but more commonly there i> 

 so little difference between the successive segments, after the spire 

 lias made two or three turns, that the breadth of each \vhorl scarcely 

 exceeds that of its predecessor, as is well seen in the section of the 

 'la, represented in fig. 624. An intermediate condition is 



FKI. G0(i. Foraminifera: Peneroplis and Orbiculina. 



presented by Rotalia, 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 ' nautiloid ' or ' turbinoid,' the former designation being 

 applied to that fonn 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 fonn in which the spire passes obliquely round 

 an axis, so that the shell becomes inequilateral,' having 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 Polystomella (Plate XIX, fig. 23) and Xoiiionina ; 

 whilst of the latter we find a typical representative in Rotalia 

 Jjeccarii (fig. 22). Further, we find among the shells whose increase 

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

 degree in which the earlier convolutions are invested and concealed 

 by the latter. In the great rotaline group, whose characteristic 

 form is a turbinoid spiral, all the convolutions are usually visible. 

 at least on one side (fig. 17) ; but among the nautiloid tribes it more 

 frequently happens that the last-formed whorl encloses the preceding 



