M. K. A. Zittel on Fossil Hexactinellida. 2G1 



that tlieir close affinities have been hitherto entirely misundc]- 

 stood by palaeontologists. The chief cause of this consists in 

 the former defective, purely macroscopical method of investi- 

 gation adopted by palaeontologists. On the other hand, 

 however, it must not be concealed that, until quite recently, 

 ■with the exception of Farrea, only those forms of living 

 Hexactinellida {Hyalonema^ Fuplectella) were known which, 

 as the most highly differentiated offshoots of the entire group, 

 show the least agreement with its fossil representatives. To 

 this must be added, further, the extremely peculiar state of 

 preservation of very many fossil Hexactinellida, which of neces- 

 sity must have given rise to erroneous conceptions of tlio 

 chemical composition of these sponge-bodies. A glance at 

 palaeontological literature, therefore, even in the most recent 

 times, shows us the most divergent opinions as to the original 

 constitution of the fossil sponges, to which we have here to 

 give a close consideration. 



By the older authors (Guettard, Parkinson, Miinster, Gold- 

 fuss, &c.) the petrified sponges were regarded either as 

 transformations of horny skeletons or as originally calcareous 

 skeletons. Toulmin Smith described the original condition of 

 the Ventriculite as "membranaceous." D'Orbigny, Etallon, 

 Queustedt, Pictet, and Fromentel regard nearly all fossil 

 forms as calcareous sponges (Petrospongia). Of the authors 

 who have more recently been active in the spongological pro- 

 vince, Pagenstecher, Capellini, Eosen, Sinzow, Harvey Holl^ 

 Kayser, &c. adopt the opinion that the fossil skeletons arc 

 calcareous or siliceous pseudomorphs of a body originally com- 

 posed of horny fibres. Only F. A. Romer and A. Pomel 

 ascribe a siliceous aivcleton to at least a number of fossil 

 forms. 



This uncertainty as to the original chemical constitution 

 may well excite our surprise, especially as we have to do with 

 a substance so resistant as silica. That certain sponges from 

 the White Chalk of England and North Germany {e.cj. from 

 the Quadratus Marls of Ahltcn, Cocsfcld), and especially from 

 the ]\Ialm of the Franconio-Swabian Jura can be freed from the 

 containing rock by treatment with dilute muriatic acid so 

 completely that the skeletons appear quite clean, as if just 

 taken out of the sea, has long been known, at least for the 

 Jurassic forms. But these beautiful skeletons were generally 

 none the less regarded as products of the metamorphosis of 

 horny or calcareous sponges. The reason for this assumption 

 was, that in the deposits which contain such s])ongcs many 

 other fossils Avith originally calcareous shells (^loUusca and 

 Echinodcrms) generally occur silicificd. Moreover it was 



Ann. & Mag, N. Hist. Scr. 4. Vol. xx. 19 



