REPORT ON THE TETRACTINELLIDA. 
415 
“ Sponge typically corticate. Skeleton usually more or less radiately arranged, with a 
dermal crust of spicules, which may be either of the same form, as in the main skeleton, 
or of a special form (spined spirulse, &c.). Megasclera typically tylostylote. Micro - 
sclera, when present, belonging almost invariably to the stellate group ; never chelae or 
sigmata. Spongin usually absent.” 
A great deal depends on the exact meaning of the word “ typical if, as I imagine, 
it is to be understood as an equivalent to “most usually,” then it will be found that 
there is not a single character in the whole definition which can be maintained as 
absolutely constant. This is not the fault of the framers, but of the group on which it 
is framed. Its value as a definition can be still better exemplified by applying it to 
some particular instance, let us say Suherites domunculus, the typical species of the 
genus Suberites, which stands as the first genus of the Clauvulina. The cortex of 
this is very poorly expressed, no better than that of some Desmacidine species ; 
the arrangement of the spicules is rather less than more radiate and might 
just as well be termed reticulate ; the spicules are, however, tylostyles, and in the 
cortex though nowhere else are arranged in the typical Suherite fashion. In this 
sponge the most valuable characters are the presence of tylostyles and their radial 
position in the cortex ; by these marks it is recognised at once as a Suherite, and in an 
exceedingly closely allied species we find centrotylote microstrongyles, — microscleres 
which are always derived from some astral form. The form of the mierosclere in 
Suberites is thus traceable to an astral ancestor, and consequently Eidley and Dendy 
might have dispensed with “almost,” and have written without qualification that the 
miscrosclere in the Clauvulina can “ invariably” be traced to an astral origin. If now 
we extend our survey from the genus Suberites we shall find that the tylostyles are 
inconstant, in the allied genus Stylocordyla they are replaced by oxeas, and in the 
Spirastrellidse we frequently encounter diactinose in place of monactinose megascleres. 
The radial arrangement of the cortical spicules further is inconstant, since in the 
SpirastreUidse they are tangentially disposed. Thus the only character which is 
constant whenever present is that of the mierosclere. 
I now give in tabular form the proposed classification;— 
Order MONAXONIDA. 
Suborder I. ASEMOPHOEA. 
With a single family. 
Family I. Homoraphida:, Ridley and Dendy. 
Homorrhaphidai, Ridley and Dendy, Report on the Monaxonida, Zool. Chall. Exp,, part lix. 
