M. K. A. Zittel on Fossil Lithistidm. 119 



acts as a separate stomachal cavity. In favour of the latter 

 assumption we have the fact that sometimes, in one and the 

 same genus, together with cup-shaped species, there occur 

 flat forms without any central cavity, in which the canals 

 furnished with oscula manifestly serve as stomachal cavities. 

 From such examples it may be seen that in the Lithistidse, as 

 in all sponges, the limitation of the individual is very uncer- 

 tain and incomplete, and therefore can only be employed with 

 caution in classification. 



To the doubtful types of cup-shaped form (in which the 

 question of individuality may be decided in either sense, 

 according as we regard the sponge-body as a strobiloid stock 

 or a simple person) the following genera belong: — Verrucu- 

 lina^ AmpMthelion^ Epistomella^ Leiodorella, Hyalotragos, 

 Azoinca, Macandrewia, Corallistes, Leiodermatium^ Gallo- 

 pegma^ &c. 



If the presence of a simple stomachal cavity appears doubt- 

 ful even in the vasiform Lithistidse, it is certainly entirely 

 wanting in a number of laminar, nodular, or disciform Lithis- 

 tidse, in which one or both surfaces are furnished only with 

 small orifices, or even only with fine pores, from which fine 

 canals penetrate more or less deeply into the sponge-body. 

 These pores perform exactly the same part as the oscula in 

 the preceding group, and may consequently be regarded either 

 as the stomachal cavities of distinct individuals of a polyzoic 

 stock, or as the mouths of canals of a simple irregular-shaped 

 sponge-body. To this category we may refer the genera Cho- 

 nella, Seliscothon^ Chenendopora^ Hagadinia^ &c. 



Finally, in a last group of Lithistidai complete astomism 

 prevails. The whole sponge-body consists of a loose, uniform 

 tissue of skeletal elements, in the interspaces of which the 

 circulation of water takes place without the aid of canals or 

 stomachal cavities. The fossil genera PZa<_?/cAo?2 2.a, Lecanella, 

 BoUdium^ Mastosia^ and Spongodiscus furnish examples of this 

 kind among the Lithistidge. 



Canal-system. 



The water-circulatory system in the Lithistidse presents 

 greater variation than in the Hexactinellidas, and even exceeds 

 in multifariousness that of the Calcispongise. From the com- 

 pact and thick- walled nature of most Lithistid skeletons, the 

 conduction of water could generally be effected only by the 

 foi'mation of definite passages which remained free from 

 skeletal elements. Then, as the latter were deposited around 

 these constant aquiferous tubes, there was produced finally a 

 regular lapidification of the canal-system, which enables us in 



