14 INTEODUCTION. 



structure are quite valueless. Manzoni has referred the Lithistids discovered by 

 him to Astrocladia, Siphonia, Jerea, and ChenendojMra, and the Hexactinellids to 

 the genus Craticularia. Some of these Miocene sponges retain their siliceous 

 structure ; in others it has been replaced by calcite. 



Classification. 



To the obliteration and alteration produced by fossilization in the structure of 

 fossil sponges may be attributed the various conflicting opinions which have been 

 held respecting their characters and the relationship between them and existing 

 forms. It is unnecessary here to refer in detail to these opinions, or to the many 

 ineff'ectual attempts which have been made to bring the numerous fossil examples of 

 the class into a natural arrangement ; it may suffice to state that all the systems of 

 classification, based on diff'ereuces of external form, or the disposition of the internal 

 canals of the sponge, have proved utterly valueless, and the heterogenous forms 

 grouped together by the advocates of these systems clearly showed their artificial 

 character. The honour of discovering a natural principle of classification for fossil 

 sponges, and thus introducing order where previously a discreditable chaos prevailed, 

 is due to Professor Zittel, who adopted, as a primary basis of classification, the 

 characters of the minute spicular bodies of which the sponge-skeleton is composed. 

 As the result of a thorough microscopic research into the minute structure of most 

 of the known fossil sponges, Zittel brought these forms for the first time into a 

 definite systematic position ; so that students of this group have now no difficulty in 

 ascertaining the relative affinity of any specimen which retains even but slight traces 

 of its original structure. The first step in arranging a series of fossil sponges in 

 natural order is to ascertain the characters of the spicular skeleton ; and as in the 

 majority of examples no spicular structure is preserved on the outer surface, it is 

 necessary to make a section through the sponge in order to discover, if possible, any 

 indications of structure in the interior. It sometimes happens that all traces of the 

 spicular skeleton have disappeared throughout the central portions of the sponge as 

 well as on the outer surface ; and in this case the systematic position of the sponge 

 remains somewhat conjectural. But even when all structure has disappeared from 

 the sponges of certain horizons and localities, we oftentimes find the same sponges 

 from the corresponding strata in other places with their skeletal structures in good 

 preservation. Owing to this fact it is possible to ascertain the original character of 

 many of the sponges from the Upper Chalk of Flamborough and the southern 

 counties of England, in which merely the outer form and canal-structure is retained, 

 by comparing them with the sponges from the same geological horizon in Northern 

 Germany, in which the spicular skeleton remains intact. 



The various modifications of the canal-system rank next in importance to the 

 spicular skeleton in afi"ording characters for the minor subdivisions of the sponges ; 



