260 M. K. A. Zittel on Fossil UexactinelUda. 



Lithistid genera. The majority of the anchorates and quadri- 

 and octoradiates figured in pi. vi. maj also belong to the 

 same order. 



In general, I am now inclined to adopt Carter's views, and 

 to ascribe to a multifarious origin the free siliceous structures 

 which occur, probably as accidental additions floated into the 

 Coeloptychian framework. By this means, as a matter of 

 course, the basis is taken away from my systematic conclu- 

 sions*, founded upon the nature of the free spicules. 



O. Schmidt, in a phylogenetic Table f? derives the living 

 Hexactinellida from the fossil Ventriculitidte. If the latter 

 denomination is used merely to express a difference of age, 

 there is nothing to be objected to this. But as a systematic 

 conception the Venticulitidte, at least in the sense adopted by 

 O. Schmidt, must disap])ear, as they in every respect belong 

 to the true Hexactinellida. 



Nor can the so-called " Vermiculatae," which 0. Schmidt 

 regards as the precursors of the Lithistida, be with any more 

 propriety separated from the latter. The designation Vermi- 

 culatfe, moreover, would have to be suppressed for the further 

 reason that among the fossil sponges " with vermiform tissue " 

 there are not only true Lithistida, but also Calcispongi^ with 

 anastomosing fibres. 



Whether the Lithistida have proceeded as a lateral branch 

 from the Hexactinellida, as W. Marshall :j: seeks speculatively 

 to show to be probable, must for the present remain an open 

 question. From the palaontological point of view scarcely 

 any thing can be adduced in favour of this liypothesis ; for, 

 although little that is reliable has been published on the dis- 

 tribution of the fossil Lithistida, yet they were certainly by 

 no means first developed in the Cretaceous period, as Marshall 

 supposes. I am acquainted with many typical Lithistida from 

 the Jura; nay, they even appear as an independent stock 

 {Aidocophim) , side by side with the Hexactinellida, as early as 

 the Silurian epoch. 



Hitherto, therefore, the investigation of living and fossil 

 Hexactinellida appears to demonstrate a sharp demarcation on 

 all sides of this order. 



State of Preservation of the Fossil Hexactinellida. 



Considering the surprising morphological agreement of 

 many fossil and recent forms, it must appear rather strange 



* Loc. cit. p. /33. t Loc. cit. p. 83. 



X " Ideen iiber die Verwandtscliaftsverh. der Hexactinelliden," Zeitsclir. 

 fiir wiss. Zool. xxvii. p. 134. 



