cx 
THE VOYAGE OF H.M.S. CHALLENGER. 
spicular fibres, and in possessing as its only megasclere an exceptional form of spicule, 
the amphitrisene, which so far as I knew at the time does not occur in any other group 
of Sponges ; while, however, I was debating as to the amount of trust to be reposed in 
the presence of the sigmaspire, I received from Mr. Carter a fragment of a new species of 
Tetilla {Tetilla stijpitata, Carter) in which an irregular form of amphitrisene is present, 
and thus the suggested alliance to the Tetillidse was confirmed. I now associate the 
Tetillidse and the Samidse (a new family of which Samus is the only representative) as 
a suborder, the Sigmatophora. Outside the Sigmatophora the sigmaspire is not met with 
in any group of Choristida, but it occurs in the Lithistida, and wdl be referred to again 
in the account of that order (p. 316). 
The next family of the Choristida is the Theneidse, which like the Tetillidae appears 
to be a very natural one, the chamber-system is eurypylous, and the mesoderm 
collenchymatous throughout, the megascleres are oxeas and triaenes in one genus 
(Thenea), and oxeas, trisenes, and calthrops in another [Pcecillastra) ; in Thenea these 
spicules are arranged in radial fibres, but in Pcecillastra they form irregular tracts, and 
the calthrops are associated with the spicules of the interior, i.e., in addition to trisenes 
situated with their cladomes in the ectosome there are calthrops mingled with the oxeas 
of the choanosome, and that as far from the ectosome as the very middle of the Sponge. 
Notwithstanding this important difference, which would have removed Pcecillastra from 
the Trisenina to the Tetradina, when these groups existed, had their definition been 
rigidly adhered to, — notwithstanding this, we find that the microscleres are remarkably 
similar, and so much so as to make any wide separation of the two genera impossible, for 
not only is the characteristic spiraster present in both, but the equivalent groups of 
microscleres also ; thus in Thenea, in addition to the spiraster, which may be modified 
into the very similar amphiaster, there are usually present metasters, and either 
plesiasters or euasters, and with the latter as varietal modifications may be associated 
mieroxeas ; in Pcecillastra, in addition to the spiraster, which in the closely allied genus 
Characella is represented by an amphiaster, there is usually present a metaster, some- 
times plesiasters, and always and most abundant mieroxeas, which represent the euaster or 
plesiaster of Thenea. In Sphinctrella a similar association of microscleres occurs, and we 
may sum up the distribution of the microscleres in the Theneidae in the statement that 
the spiraster is present in all species or represented by the amphiaster ; metasters a,re 
generally present ; plesiasters or euasters are present in all species of Thenea, but in the 
remaining genera of the family are represented by mieroxeas, i.e., diactinose asters. 
Having found that the characters of the spiraster are as constant in the Theneidae as those 
of the sigmaspire in the Tetillidae, and that it possesses the additional recommendation 
of constancy in presence as well as in form, we shall look upon it as a very promising 
guide, still, however, to be followed by caution ; thus in several genera which I have 
placed in the Euastrosa — Stryphnus for example — amphiasters occur, but these are of 
