314 M. F. Dreyer on the 



external, continuous shell-closure is developed only after the 

 completion of the growth of the spongy disk *. 



As we have seen, the agglutinated and calcareous materials 

 agree in that, as compared with the silica, they possess 

 less firmness, the consequence of which is that the Thalamo- 

 phoran shells are more compactly and simply constructed than 

 the siliceous skeletons of the Radiolaria. On closer exami- 

 nation, however, a distinction may be recognized between the 

 agglutinating and calcareous Thalamophora, consisting in the 

 fact that the former are more coarsely and simply constructed 

 than the latter, and this is certainly due to the agglutinated 

 constructive material being inferior in solidity to the homo- 

 geneous calcareous mass. Although this difference is not so 

 great as that between Thalamophoran and E-adiolarian shells, it 

 nevertheless exists, and to all appearance its importance must not 

 be undervalued. Quite recently Neumayr has specially called 

 attention to this circumstance, and made use of it for a phylo- 

 geny of the Thalamophora, assuming the more highly ditFer- 

 entiated calcareous -shelled forms to have become developed 

 from the simple arenaceous-shelled types as their stem-forms f. 

 It will be most convenient, in the first place, to reproduce this 

 theory of Neumayr's in the author's own words. He says : — 

 " The low forms furnished with the most imperfect shell- 

 structure which form Brady's very well - founded family 

 Astrorhizidse are exclusively sandy ; the most highly deve- 

 loped Foraminifera, furnished with a branched canal-system, 

 double septa, an intermediate skeleton, &c., are exclusively 

 calcareous ; while the forms standing between the two are 

 partly sandy, partly calcareous, and show many transitions 

 from one development to the other. This condition of things 

 leads to the supposition that arenaceous forms, without any 

 trace of a complicated structure, such as we find in the Astrorhi- 

 zidse, represent the stem-types from which the other Forami- 

 nifera have been developed. ... In favour of the notion that 

 the arenaceous Foraminifera in reality represent the original 

 type, we have in the first place their geological occurrence, 

 inasmuch as they occur in old deposits in comparatively much 

 greater number than subsequently ; it is true that in the com- 

 parison of the living with the Tertiary and Mesozoic species 

 this does not appear so strikiiigly, but it is perfectly distinct 



* See for further details my ' Pylombildungen,' Abschnitt v. Taf. v. 

 figs. 64-69, and Taf. vi. figs. J)7-100. 



t Neumayr, " Die uatiirlichen Veiwandscliaftsverhaltnisse der schalen- 

 tragenden P'oramimferen," in Sitzungsb. Wien. Acad. Bd. xcv. Abth. 1 

 (1887), and also in ' Die Stamme des Tbierreichs,' Bd. i (1889). 



