REPORT ON THE TETRACTINELLIDA. 
CXVli 
than approximations, and how much less if we found our classification on the fossil rather 
than on the recent Sponges ; in the former the microscleres are inevitably lost in the 
process of petrefaction, so that not a single fossil Sponge has yet yielded a trace of these 
important guides, and the characters of the chamber-system, only next important, must 
necessarily disappear with the soft parts. 
The most important observation made by Schmidt bearing on Zittel’s classification is 
the existence of a passage from the Tetracladine type of desma to the Rhizomorine, and 
this I can fully confirm from my own examination of Schmidt’s material ; the passage 
occurs in Macandrewia clavatella,^ and clears up many difficulties as regards the 
relationships of the Lithistids, a subject that will be discussed directly. 0. Schmidt^ 
having found that the distinction between the Rhizomorina and the Tetracladina cannot 
be maintained as absolute, laid great, and as I think undue, stress on another, the 
presence, namely, of the discotrisene, which he made the character of what he named the 
“ Discodermia ” series ; the presence of the discotrisene does not correspond either with 
generic or family distinctions, and can no more be made use of as a classificatory 
character than say the dichotrisene, as distinguished from the orthotrisene, in the case of 
the Choristida. 
Too little is known of the characters of the soft parts in the Lithistida to enable us to 
judge how far they may be useful in classification. In the following five species, Theonella 
swinhoei, Discodermia discifurca, Corallistes typus, Corallistes masoni, and Pleroma 
turbinatum, the chamber system is aphodal, and the choanosomal mesoderm sarcen- 
chymatous, in Azorica pfeifferse the chamber system is diplodal and the choanosomal 
mesoderm of an exceptional character {vide p. 321). The difference between an aphodal 
and diplodal chamber system in the Lithistida is not very marked, and is not of the 
same importance as that between the aphodal and eurypylous systems. It is of interest, 
however, to observe that the species enumerated as possessing an aphodal system are 
more closely related to one another than to Azorica, which is widely separated from 
them on other grounds. The size of the chambers in six species, viz., Theonella 
swinhoei, Discodermia discifurca, Corallistes typus, Corallistes masoni, Siphonidium 
capitatum, and Azorica pfeifferw, is very similar, ranging from 0‘015 to 0'024 mm. in 
length, by 0*018 to 0’031 mm. in breadth; in Pleroma it is exceptionally 
great, viz., 0*04 by 0'044 mm. 
The characters on which we must depend for the subdivision of the order are those of 
the skeleton, including of course the microscleres, the presence or absence of which can 
be definitely ascertained in most of the existing Lithistida, though never in the fossil 
forms. 
^ 0. Schmidt, Spong. Meerb. v. Mexico, p. 24, 1879. 
2 Loc. cit. 
