GENERAL CHARACTERS OF FORAMIXIFERA. 485 



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 ' Nauti- 

 loid ' 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,' 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 characteristic examples in 

 Polystomella (Plate xv., fig. 16) and Nonionina (fig. 19) ; whilst 

 of the latter we find a typical representation in Rotalia Beccarii 

 (fig. 18). Further, 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 later. In the great Rotaline group, whose characteristic 

 form is a Turbinoid spiral, all the convolutions are usually visible, 

 at least on one side (Plate xv., figs. 15, 17, IS) ; but among the 

 Nautiloid tribes it more frequently happens that the last-formed 

 whorl encloses the preceding to such an extent that they are 

 scarcely, or not at all, visible externally, as is the case in Cris- 

 tellaria (Plate xv., fig. 11), Polystomella (fig. 16), and Nonionina 

 (fig. 19). — The Turbinoid spire may coil so rapidly round an elon- 

 gated axis, that the number of chambers in each turn is very small ; 

 thus in Globigerina (Plate xv., fig. 12) there are usually only four ; 

 and in Valvulina the regular number is only three. Thus we are 

 led to the biserial arrangement of the chambers which is character- 

 istic of the Textularian group (Plate xv., fig. 14) ; in which we find 

 the chambers arranged in two rows, each chamber communicating 

 with that above and that below it on the opposite side, without 

 any direct communication with the chambers of its own side, as 

 will be understood by reference to Fig. 253, a, which shows a • cast ' 

 of the sarcode-body of the animal. — On the other hand, we find 

 in the Nautiloid spire a tendency to pass (by a curious transitional 

 form to be presently described, § 374) into the Cyclical mode 

 of growth ; in which the original segment, instead of budding 

 forth on one side only, puts forth gemma all round, so that a 

 ring of small chambers (or chamberlets) is formed around the 

 primordial chamber, and this in its turn surrounds itself after the 

 like fashion with another ring ; and by successive repetitions of the 

 same process the Shell comes to have the form of a disk made up 

 of a great number of Concentric Rings, as we see in Orbitolites 

 (Fig. 250) and in Cycloclypeus (Plate xvi., fig. 1). 



368. These and other differences in the plan of groivth were 

 made by M. D'Orbigny the foundation of his Classification of this 

 group, which, though at one time generally accepted, has now 

 been abandoned by most of those who have occupied themselves 



