272 M. Iv. A. Zittel on Fossil Hexactinellida. 



fashion that they ajyparenthj form a single tube, although they 

 do not actually coalesce. But this renders the section Synau- 

 loidaj superfluous*, and Sderothamnus enters the group Pleio- 

 nakidse. 



There remain therefore the three sections of the Monakidie, 

 Pleionakida3, and Pollakid^e, which are based uj)on the greater 

 or less differentiation of the parts of the skeleton. 



Whether there are any Monakidas among living Hexacti- 

 nellida seems to me to be doubtful. Of the only genus referred 

 to this position, namely Eurete, Semp., only a " very much 

 bleached and washed specimen" exists. Now, as the con- 

 nected siliceous skeleton agrees perfectly with Farrea, and 

 Bowerbank f has proved that in Farreajistulata (which is per- 

 haps identical Avith Eurete simplicissimaj Marsh.) " spicula are 

 present in great numbers wherever the skeleton is covered 

 witji dark brown sarcode, but that not a single spicule is to 

 be seen where the sarcode is "wanting," I regard the group of 

 the Monakidaj in Marshall's conception as doubtful. 



To the palaeontologist every classification founded upon the 

 free spicules is useless, as only in very rare cases can the 

 connexion between Hexactinellid skeletons and the neigh- 

 bom-ing " flesh-spicules " be proved. Moreover, as I have 

 already shown in my monograph on Cosloptycliiwnj uncom- 

 monly few spicules of the Hexactinellid type occur among 

 the free fossil siliceous structures. Especially I have never 

 been able to detect any trace of " rosettes, fir-trees, and broom- 

 forks," and of the otlier certainly very minute and very fragile 

 forms upon which Carter particularly relies in the separation 

 of his genera. 



If, however, a preponderant systematic importance is given 

 to the " flesh-spicules," not only the fossil Hexactinellida, but 

 also all those existing forms, the skeletons of which are washed 

 and no longer clothed Avith sarcode, remain undeterminable. 



But, leaving out of consideration this practical objection, 

 there are internal reasons opposed to any such principle of 

 classification. The flesh-spicules form, to a certain extent, 

 the external adornment of the sponge-body ; they may be 

 compared with the plumage of birds and the dermal coverings 

 of fishes, reptiles, and mammals. If we take our stand on the 

 ground of the transmutation theory, we certainly have before 

 us in the flesh-spicules those parts of the sponges which change 

 most readily by adaptation, and therefore most easily throw oif 



* Dr. Marshall, as he informs me by letter, has also convinced himself 

 of this fact, and will withdraw the Synavdoidae in a memoir very shortly 

 to be published. 



t Op. cit. part iii. (April 6tb, 1875) p. 276. 



