lituollda; arenaceous tests. 499 



form a series of their own. The genus Trochammina in its 

 simplest form represents the undivided spiral Cornuspira among 

 the Porcellanous, and Spirillina among the Vitreous Foraminifera; 

 but besides presenting a number of other curious varieties of form, 

 it exhibits in some instances such a tendency to the subdivision of 

 its tube into chambers, as to approach the lower and less regular 

 forms of the Rotaline series in its plan of growth. The 'test' of ] 

 Trochammina is very fine in its texture, the cemented particles ' 

 being very minute. That of Lituola, on the other hand, is very | 

 coarse, especially in those large fossil specimens which are very f 

 common in the Chalk. The typical shape of this genus, from 

 which it derives its name, very much resembles that of the 

 ' spiroline ' Peneroplis (§ 373), being a nautiloid spiral partly 

 unrolled; but its lower forms are often very irregular, growing 

 adherent to Shells, Corals, Stones, &c. , and losing all definiteness 

 of plan, .not unfrequently even branching and spreading themselves 

 out in different directions. On the other hand, Lituola sometimes 

 presents itself as a simple Nautiloid spiral, so exactly resembling 

 Nonionina (Plate xv., fig. 19) as to have been mistaken for it, 

 though at once to be distinguished by the nature of its 'test.' 

 In the largest and most developed Fossil forms, especially those 

 having a ' spiroline ' mode of growth, the chambers are irregularly 

 subdivided into ' chamberlets ' by secondary septa, which, like the 

 principal septa, are formed of the same material as the external 

 walls of the 'test,' namely, particles of Calcareous or Siliceous 

 sand, cemented together by a sort of mortar composed of finer 

 particles united by an Organic exudation. Still more remark- 

 able Arenaceous ' tests ' of gigantic size have been lately 

 discovered ; some of them resembling Alveolince in plan of 

 growth, whilst others are Spheres made up of concentric layers. 

 The Physiological relationship of the foregoing is obviously to 

 the Porcellanous Foraminifera ; since, as the walls of the ' test ' 

 are not perforated for the passage of Pseudopodia, the Sarcode- 

 body enclosed in them has no other communication with the 

 exterior than through the aperture of the last segment. There i 

 is, however, another Arenaceous genus, Valvulina, in which the I 

 test has a true shelly basis, perforated with distinct pores, and I 

 therefore leading us towards the Vitreous series. The characteristic ' 

 form of this Genus resembles that of Textularia (Plate xv., fig. 14), 

 except that the chambers are piled one on another in three 

 series (so as to form a kind of pyramid having the primordial 

 chamber at its apex) instead of in tivo ; the aperture, however, is 

 furnished with a valvedike flap or tongue, somewhat resembling 

 that of Miliola (Plate xv., fig. 4). But this form is subject to a 

 great variety of modifications; the chambers, for example, being 

 sometimes arranged in a Bulimine spiral, whilst sometimes they 

 are attached end to end, so as to constitute but a single straight 

 series, thus giving a ' clavuline ' or nail-like form to the shell. 



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