2G8 M. K. A. Zittel on Fossil EexactinelUda. 



and Litliistida *. This distinguished Spongologist in the first 

 place points out tlie distinction between the " skeleton-spi- 

 cules," which form the true siliceous skeleton and usually 

 possess a tolerably uniform character, and the so-called "flesh- 

 spicules," which are always freely imbedded in the sarcode 

 and are generally remarkable for their minute size and extra- 

 ordinary multiplicity of form. 



Carter divides the Hexactincllida into the following- three 

 groups t :— 



1. ViTEEOHEXACTiNELLiDA. Spiculcs united by silicified 

 fibre. 



Dactylocalyx, Myliusia, Eujylectella asjjercjillum^ Apliro- 

 calUsteSj Aulodictyon, Fcirrea^ Sympagella. 



2. Saecohexactinellida. Spicules united by amorphous 

 sarcode. 



Ashonema, Crater omorjplia, Ilossella^ Hahrodictyon^ Hyalo- 

 nertia, Pheronema, Meyerina. 



3. Sarco-yitreohexactinellida. Spicules united partly 

 by siliceous fibre, partly by amorphous sarcode. 



Euplectella cucumer. 



The first two families break up into several subdivisions, 

 characterized partly by the external form of the sponge-body 

 and partly by the nature of the " flesh-spicules " and especially 

 of the so-called " rosettes." 



It will be seen that Kent and Carter, in the limitation of 

 their primary groups, lay the first importance on the question 

 whether the skeleton consists only of isolated spicules, or the 

 different parts are amalgamated with each other and form a 

 coherent framework. But, many as are the advantages that 

 Carter^s classification possesses over that of Saville Kent, it 

 cannot satisfy us when forms so evidently allied as Euplectella 

 aspergillum, Hahrodictyon, and Eupilectella cucumer are placed 

 in three different families, A further defect of Carter's system 

 appears to me to be that too much importance is ascribed to 

 the nature of the flesh-spicules, and too little to that of the 

 true skeleton. 



W. Marshall's first memoir J is divided into a general and a 

 special part. In the former Marshall in the first place gives a 

 critical revision of the previous works on the Hexactincllida, 

 then a description of their external form and distribution, 

 which he follows with a very careful description of the sarcode 



* Ann. & Mag. Nat. Hist. 1873, ser. 4, vol. xii. pp. 349 & 437. 



t Ibid. 1873, xii. p. 367, and 1875, xvi.p. 199. 



X '• Unteisuchungen liber Hexactinelliden," I. c. Bd. xxv. 



