258 M. K. A. ZIttel on Fossil Hexactinellida. 



trea) of Wyville Thomson included two fundamentally different 

 types, the Hexactinellida and Lithistida, each of Avhich pos- 

 sesses numerous fossil precursors, that a foundation upon 

 which one may go on building was afforded to palaeontology. 



As I have already shown in a previous memoir*, the sug- 

 gestions of these naturalists were but little attended to by 

 palaeontologists, who continued as before to stick to the unsuc- 

 cessful systems of D'Orbigny and Fromentel ; and although 

 timid attempts had already been made by Etallonf, F. A. 

 Romer:|:, and most recently by Pomel§ to take into account 

 the structural conditions even in fossil sponges, these remained 

 without results, owing to the macroscopic method of investi- 

 gation which had been hitherto almost exclusively employed. 



In the meanwhile the knowledge of the recent Hexactinel- 

 lida and Lithistida has been so essentially advanced by Carter||, 

 W. Marshall^, Saville Kent**, Bowerbankft, WyviUe Thom- 

 son I:]:, Wright §§, and others, that these orders of sponges, 

 comparatively late as was their discovery, are now amongst- 

 the most carefully studied. 



As to the separation of the Hexactinellida and Lithistida, 

 which had been united by Gray as Coralliospongia, by W. 

 Thomson as Vitrea, and by Bowerbank as " Siliceo- fibrous 

 Sponges," no noteworthy difference now exists between most 

 students of living sponges. The differentiation of the two 

 orders is indeed remarkably sharp, and may be carried out 

 with equal certainty for the fossil forms. 



In the Hexactinellida the siliceous skeleton consists of 

 elements founded almost without exception upon tln-ee axes 

 crossing each other at right angles ; while in the Lithistida 

 the axes generally come together at an angle of 120°||||, and in 

 this way chiefly form quadriradiate bodies, which are united 



* " Ueber Coeloptvchium," Abb. d. k. baj-er. Akad. II. CI. Band xii. 

 Abtb. iii. 1876. 



t 'Actes de la Societe Jurassienne d'emulation pendant 1858,' Porren- 

 truy, 1860, p. 129. 



X " Die Spongitarien des norddeutscben Kreidegebirges," Palseontogra- 

 pbica, xiii. 1864. 



§ Paleontoloo-ie de la Pro\ince d"Oran, 5^ fasc. Spongiaires, 1872. 



il .\im. & Mag. Nat. Hist. 4tb ser. vol. xii. 1873, pp. 349 and 437. 



^ Zeitscbr. fiir wiss. Zool. Bd. xxv. Suppl., and Bd. xxvii. p. 113, 



** Montblj' Microsc. Journ. 1870, vol. iv. p. 241. 



ft "Monograpb of tbe Siliceo-fibrous Sponges," Proc. Zool. Soc. 1869, 

 pp. 06 and 323 ; 187o, pp. 272, /503, o58 ; and 1876, p. 535. 



XX Ann. & Mag. Nat. Hist. 4tb ser. vol. i. 1868, p. 119 ; Deptbs of tbe 

 Sea, 1873 ; and Pbil. Ti-ans. 1869, p. 701 (on Holtenia). 



§§ Quart. Journ. Microsc. Sci.vol. x. 1870, p. 4. 



II II Not 130°, as stated in my monograpb on Coeloptychium, p. 45, in 

 consequence of a misprint. 



