M. K. A. ZIttel on Ftssil Hexactinelliaa. 259 



with each other in a peculiar manner, as will be shown in a 

 subsequent memoir. 



Together with the triaxial and quaclriaxial fundamental 

 forms, uniaxial spicules occur in abundance both in Hexacti- 

 nellida and Lithistida, as well as multiaxial siliceous bodies 

 more isolatedly *. 



0. Schmidt, Carter, and Marshall find no close relationships 

 between the Hexactincllida and the other existing sponges. 

 The order is quite isolated, and shows oo transitions towards 

 other families in any direction. It is true that Saville Kent 

 at first thought he had found a point of union between Hexac- 

 tincllida and Tethyida in the genus DorvilUa f ; but it after- 

 wards turned out :|: that the six-rayed spicules observed by Kent 

 had got accidentally into the body of the sponge, and that 

 consequently the genus DorvilUa had to disappear from our 

 literature, as a synonym of Tethya. 



The fossil Hexactincllida are no less sharply separated than 

 the living ones from all other sponges, and especially even 

 from the Lithistida. This statement contradicts the opinions 

 expressed in my monograph of Coeloptychium §, which I 

 founded upon the occurrence of free siliceous structures in the 

 skeleton of Coelo])tyclnum. I believed then that, from the oc- 

 currence and the state of preservation of these multiform 

 bodies, I might conclude that the genus Coeloptychium pos- 

 sesses spicules of uniaxial, triaxial, quadriaxial, quinquiaxial, 

 and multiaxial type ; but the continuation of my studies on 

 fossil sponges taught me that it is only in rare cases that the 

 free siliceous structures are still so intimately united to the 

 coherent skeleton that they can be made available for syste- 

 matic purposes. 



In certain localities we frequently find perfectly different 

 sponge-bodies filled with the same spicules ; and an examina- 

 tion of the surrounding rock almost always shows a super- 

 abundance of the corresponding structures. Petrogenetically 

 sponge-spicules in general play a much more important part 

 than has hitherto been supposed ||. 



As regards the free siliceous spicules figured in my mono- 

 graph of C<£loj)tychium, I believe now that I may refer the 

 forms represented on pi. vii., and especially the irregular 

 ones Avith the short axial canals, for the most part to definite 



* The latter have been demonstrated with certainty f nlv in Lithit.tida. 

 t Monthly IVIicroBc. .Tourn. 1870, p. 293. 



t Ann. &"'Mag. Nat. Iliat. 4th ser. vol. vii. 1871, p. 37, and vol. x. 1872, 

 p. 200. 



§ Loc. at. pp. 34, 49, o3. 



't See also Kutot, Ann. de la Soc. Malacologique de Belgique, ix. 1874. 



