REPORT ON THE TETRACTINELLIDA. 
CXI 
such irregular form that they may be of different origin to those of the Theneidse, and 
so for the present I leave the genus with what appear to be its more natural 
associates. 
As an instance of the application of the character, I may select the new genus 
Triptolemus, first known to me, like Samus, by a single exceptional form of spicule, the 
centrotrigene, which is confined to this genus ; a slide showing all the spicules, however, 
was given me by Mr. Carter, and the presence of microxeas and spirasters showed at once 
its relationship, which appears natural enough directly it is pointed out. 
The family Pachas trellidse which will next engage our attention is of great interest 
in this inquiry. The typical species, Pachastrella ahyssi, like all the species of the 
family, is characterised by a sarcenchymatous mesoderm, aphodal chamber-system, and 
choanosomal calthrops ; in the two former characters it differs from the Theneidse, in the 
latter it resembles the Theneid genus Pcecillastra, but its resemblance to Pcecillastra 
is still further increased by the complement of microscleres, for these are spirasters, 
microxeas, and microstron gyles, all but the last named being evidently correspondent to 
those of Pcecillastra, while the microstrongyle is readily explicable as a reduced aster. 
Pachastrella ahyssi may be regarded as a Pcecillastra which has attained a higher plane 
as regards the chamber-system and the mesoderm, and in which some of the microxeas 
have become shortened into microstrongyles. If now we pass to the other genera of 
Pachastrellids we shall find our caution as to the treacherous nature of single characters is 
no libel ; so far as the spicules are concerned the only constant form is the calthrops ; the 
microscleres vary unintelligibly, spirasters may be rejDlaced by euasters [Calthropella), 
the microxeas disappearing, or instead of any astrose form, toxas and microrabds may be 
present [Dercitus), and in this genus the oxea of the megascleres also vanishes. With 
these wide differences to explain, we may suppose (1) that great variation occurred within 
the group after its separation from the Theneidse, or (2) that it is of polyphyletic origin, 
i.e., not a natural family ; if we trust to the characters of the mesoderm, of the (chamber- 
system, and to the presence of the calthrops, we shall incline to the first alternative, if to 
the microscleres to the second ; I am myself in favour of the first, the strongest objection 
to it being furnished by the microscleres of Dercitus, for as regards the aster of 
Calthropella, that may well be derived from a spiraster, but the toxa and spinose 
microrabds of Dercitus are less readily explained, and instead of speculating upon them, 
it may be as well to wait for information as to their embryological development, which 
ought not to be difficult to study in the case of a not very uncommon British Sponge. 
That they do not offer an insuperable objection, however, I feel convinced, since there are 
several possible ways in which the microscleres of Pachastrella ahyssi may have produced 
them. 
Considering the two families — Theneidse and Pachastrellidse — together, we conclude 
that the spiraster is present in most of their genera, they may therefore be united as a 
