M. K. A, Zittel on Fossil Lithistidce. 243 



Billings, Courtiller, Poinel, &c., because the extant descrip- 

 tions and figures furnish no information as to the minuter 

 structural characters. 



The Silurian genus Aulocojnum presents some difficulties. 

 Its skeletal elements are of indifferent type : in many respects 

 they resemble those of the Tetracladina ; but their irregularity 

 approximates them equally to Anomocladina, and even to 

 certain Rhizomorina. This is a significant hint as to the 

 history of the development of the Lithistid stem. In the 

 Silurian formation the different groups appear not to have 

 yet acquired the differentiation which they show later on ; 

 the characters which subsequently divided themselves be- 

 tween the individual families are in part still united in the 

 old collective type. 



The Anomocladina also occupy a similar position. They 

 show affinities in all directions ; but it is impossible for the 

 })resent to bring them into genetic connexion with any other 

 group of Lithistidse. 



The other groups, Rhizomorina, Megamorina, and Tetra- 

 cladina, may be with great certainty distinguished from one 

 another by their microstructure, as a glance at the Tables 

 shows. The Jurassic Rhizomorina, in their short, simple, axial 

 canal, present a remarkable difference from the Cretaceous and 

 recent representatives of this group. All the Jurassic genera 

 of Rhizomorina possess very similar skeletal corpuscles ; so 

 that the nature of the skeleton alone does not suffice to cha- 

 racterize the genera. Mutatis mutandis this applies also to 

 the Cretaceous Tetracladina and Rhizomorina. 



Habitat and Distribution of the Recent Lithistidoi. 



The circumstance that the first recent form belonging to the 

 Lithistidaj {Macandrewia azorica, Gray) was described in 

 1859, proves sufficiently that these sponges are not to be 

 reckoned among the easily accessible inhabitants of our sea- 

 coasts. In fact nearly all the species at present known have 

 been brought up from the ocean by the dredge ; and the few 

 data as to their bathymetrical distribution, for which we are 

 indebted to O. Schmidt and Carter, would indicate that the 

 Lithistidee dwell between 75 and 374 fathoms, and frequently 

 occur associated with Hexactinellida^. Tlie following Table 

 contains the extant information as to their geographical and 

 bathymetrical distribution *. 



* Tlie species marked with an * are those which I liave had the oppor- 

 tiinitv of examiniiifj in nature. 



