268 M. K. A. Zittel 07i Fossil EexactmelUda. 



and Litliistida *. This distinguished Spongologist in the first 

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

 cules," which form the true siliceous skeleton and usually 

 jjossess 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. VlTEEOHEXACTiNELLiDA. Spicules united by silicified 

 fibre. 



Dactylocalyx, MyUusia^ EuplecteUa aspergillum^ ApJiro- 

 callistes, Aulodicfyon, Farreaj Symijagella. 



2. Saecohexactinellida. Spicules united by amorphous 

 sarcode. 



Ashonemaj Crateromoiyha, Rossella, Sabrodictyon, Hyalo- 

 nevia, Pheronemaj Meyerina. 



3. Sarco-viteeohexactinellida. Spicules united partly 

 by siliceous fibre, partly by amorphous sarcode. 



EuplecteUa ciicumer. 



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 

 difterent parts are amalgamated with each other and form a 

 coherent framework. Eut, 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 EuplecteUa 

 aspergillum^ Eabrodictyon^ and Eiplectella 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 

 ti-ue 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, 

 tlien a description of their external form and distribution, 

 which he follows with a very careful description of the sarcode 



* Ann. & Mag. Nat. Hist. 1673, ser. 4, vol. xii. pp. 349 & 437. 



t Ibid. 1873, xii. p. .357, and 1.875, xvi.p. 1!)9. 



X '' Unteisucliungen iiber Hesactinelliden," /. c. Bd. xxv. 



