Structure of Rhizopod Shells. 305 



poly thalamons Thalamophora, some pylomatic Spumellaria *, 

 and the Plia^odarian families Cliallengerida, Medusettida, and 

 Tuscarorida. 



Close to the eudipleural forms come the spirally-wound 

 Rhizopod shells, which are to be regarded essentially only as 

 a continuation of the eudij)leural ground-form by the process 

 of terminal growth, which will presently he referred to more 

 particularly. There are therefore, especially in freshwater 

 Rhizopoda, very gradual transitions from simply eudipleural 

 to spirally twisted shells. In this respect the Diffliigke are 

 particularly instructive, as in them all transitions from mon- 

 axonic to eudipleural and from these to spiral shells are 

 represented ; thus, for example, Diffiugia corona is typically 

 monaxonic, D. marsiqnjormis^ with the pylom displaced 

 forward, eudipleural, while, finally, D. sjnralis already shows 

 distinctly the half-turn of a spiral f. In the same way as in 

 these first and perhaps still individually varying commence- 

 ments in the freshwater Rhizopoda, the highly developed 

 marine Thalamophora, often showing many spiral windings, 

 have been developed, as is indicated, among other things, by 

 the monaxonic central first chamber (the so-called embryonal 

 chamber). 



Having in the preceding submitted the Rhizopod shell to a 

 short consideration with regard to its form^ we may now pro- 

 ceed to examine it somewhat more closely from another point of 

 view, namely as to the mode of its growth. In this, at the 

 first glance, we meet with an interesting parallelism with the 

 two form-types just referred to. Just as in the case of these 

 form- types we can also distinguish in the mode of growth of 

 the Rhizopod shell two principal types, which may be placed 

 side by side with the two form-types, and on the whole are 

 to be conceived as a continuation of the latter caused by 

 growth. Thus the perforate form-type corresponds to the 

 concentric type of growth, and the pylomatic form-type to the 

 terminal type of growth. 



The concentric growth-type, as implied by its name, con- 

 sists in that the soft body during its further growth around 

 its first spherical perforate shell, which gradually becomes 

 too small for it, separates externally successive larger con- 

 centric spherical shells. The shells of such a system of 

 latticed spheres nested one within the other are bound together 



* The bilaterality indicated in a great number of Nassellaria by the 

 relations of the basal and apical spines is original and does not belong to 

 this category. See * Radiolarienstudien,' Heft i. p. 100, note 2. 



t .See ' Radiolarienstudien,' Heft i. Taf. vi. figs. 88, 89, 90. 



Ann. & Mag. N, Hist. Ser. 6. Vol. iv. 21 



