M. K. A. Zittel on Fossil LitMstidoe. 117 



is furnished with root-like processes at the extremity ; others 

 are attached to their support by a broad base, or may even, 

 under certain circumstances, live as parasitic crusts upon 

 foreign bodies ; and only a few [Aulocopium, PUnthosella^ 

 Sponyodiscus) appear to be destitute of any point of attach- 

 ment. 



From the Hexactinellidas the Lithistidas in general differ 

 by their much thicker walls and by the denser texture of the 

 siliceous skeleton. Thin-walled tubes, or majandrically con- 

 torted delicate lamina?, such as are not unfrequently observed 

 among the Hexactinellidfe {Euplectella, Eurete, Plocoscyphta, 

 Myliusid), never occur among the Lithistidse. The sponge- 

 body consists of a compact stony mass of great solidity, which, 

 when examined macroscopically reminds one rather of the 

 structure of certain corals and Hydromedusse with highly 

 developed ccenenchyma than of that of the ordinary sponges. 



The presence or absence of one or of several stomachal 

 cavities has essential influence upon the external appearance. 

 If a single central infundibuliform or tubular body-cavity sinks 

 into a sponge-body of cylindrical, conical, globular, or pyri- 

 form shape, there can be no doubt as to the monozoic charac- 

 ter of the latter. The genera Aulocopmm, MeloneMa, Cylin- 

 drophyma^ Coehcoryphaj Scytalia^ Pachinion, Siplionia, Tra- 

 chysyeon^ Phymatella, Theonella^ Discodermia^ IsorapMnia^ 

 &c. are in this case. 



With equal certainty we may regard as polyzoic stocks 

 those forms in which isolated large oscula with corresponding 

 canal-depressions are distributed at considerable distances 

 upon a nodular or ramified body, as, for example, in the 

 genera Astrobolia and Astrocladia. 



A phenomenon very characteristic of certain fossil Lithis- 

 tid^e is the replacement of a simple stomachal cavity by a 

 greater or less number of vertical tubes, sometimes grouped in 

 bundles, sometimes arranged in series, sometimes irregularly 

 distributed, penetrating the skeletal mass of the sponge-body 

 in a perpendicular or nearly perpendicular direction, and 

 usually reaching down to the base. These tubes are gene- 

 rally round, unramified, like quills, and nearly of the same 

 diameter throughout their length, whilst the true stomachal 

 cavities are always more or less narrowed downwards. Their 

 orifices are situated in the vertex or at the upper margin of 

 the sponge-body, which in most cases possesses a cylindrical, 

 branched, or elongate-pyriform shape. In this group of 

 Lithistidai the question of their monozoic or polyzoic nature 

 is difficult of solution. Their canal-system presents exactly 

 the same characters as in tlie monozoic forms of the first 



