220 PALEONTOLOGY OF ILLINOIS. 



1. Myxospongise Haeckel. Sponges unprovided with a solid 

 skeleton. 



2. Ceraospongise Bronn. Skeleton consisting solely of horny 

 fibres. 



3. Monactinellidse Zittel. Skeleton consisting entirely of uni- 

 axial siliceous spicules, or of horny fibres enclosing an uniaxial 

 siliceous core. 



4. Tetractinellidss Marshall. Skeleton consisting principally 

 of four-rayed siliceous spicules, one usually elongated to form a 

 shaft. Uniaxial and stellate spicules are also present. 



5. Lithistidse Schmidt. Skeleton composed of four-rayed or 

 irregular siliceous spicules, which are often branchy and always 

 closely interwoven so as to form a continuous mesh. 



6. Hexactinellidse Schmidt. Skeleton composed of isolated, 

 or united and interwoven six-rayed siliceous spicules. 



7. Calcispongiae Blainville. Skeleton composed of calcareous 

 spicules, either uniaxial, three or four-rayed. 



Of these divisions five are known to be represented in the 

 palaeozoic rocks of America. 



Zittel's system, as it is based mainly upon mesozoic and recent 

 forms, fails, like Busk's and other classifications of the Bryozoa, 

 to make full provisions for the palaeozoic species, and many of 

 them require some pressure to fit them into the accurately drawn 

 divisions. However until something approaching a complete 

 series of palaeozoic sponges can be studied, it would be worse 

 than useless to propose any serious changes from the arrange- 

 ment adopted by Prof. Zittel in his "Handbuch der Palaeontolo- 

 gie," excepting in such cases where the system has been improv- 

 ed upon by himself and other authorities on the subject. 



Under this heading we desire to introduce a list of American 

 palaeozoic sponges, as nearly arranged according to the classi- 

 fication adopted as the present state of our knowledge would 

 admit. As several changes and innovations are proposed, some 

 remarks upon them are necessary to establish our claims. In 

 order that the list may be as available for reference as possible, 

 we prefer to give them here, rather than in the body of the 

 catalogue. 



