REPORT ON THE TETRACTINELLIDA. 
CIX 
others, such as Pcecillastra, Characella, and Sphinctrella. So too in the Stellettidse a 
radial arrangement is characteristic, but an exception is presented, by the genus Stryphnus ; 
in the Geodiidse also the genera Erylus, Caminus, and Pachymatisma are non-radiate. In 
the Sigmatophora the arrangement of the spicules is best known in the family Tetillidse ; 
throughout this group it is radial, but in the lower forms, such as some of the species of 
Tetilla, the radial spicules are often crossed by numerous others concentrically arranged. 
From these examples, particularly that of the Theneidse, it would appear that the 
disposition of the spicules is a character which cannot be relied on for family distinctions. 
Finally, as regards the sigmaspire, this is the most constant next to the protrisene 
with regard to form, but it is one of the least constant as to occurrence ; it, or some obvious 
modification of it, is always present when microscleres are present at all ; it may pass into 
a microstrongyle by straightening out and increasing in size, but in the only species in 
which this modification occurs the normal sigmaspire is present as well ; it may also 
develop an additional half- turn and pass into a toxaspire, the normal sigmaspire in this 
case also persisting along with its modification ; or finally it may become sparsely spined 
and thus approach the spiraster, but it is never replaced by a true aster. Thus, when 
present, the sigmaspire is characteristic ; on the other hand it may fail altogether, so 
that allied species otherwise precisely similar may differ solely by the presence or 
absence of the sigmaspire,^ e.g., Craniella cranium, in which it is present, and Craniella 
zetlandica, in which it is absent. So similar in all other respects are these two species 
that but for the absence of sigmaspires in the one they could not be distinguished ; and 
I should have been inclined to regard the absence of the sigmaspire as accidental, were it 
not that the young sponges, while still within the body of the parent, present the same 
difference as the adults, a fact first stated by Carter, and for which I can vouch, having 
seen Mr. Carter’s specimens. 
Now, while the occasional absence of the sigmaspire diminishes its value, in so far 
that it proves to be a guide which may at a pinch desert us, it does not by any means 
impeach its trustworthiness when present, and if we attempt to follow it in other groups 
of Sponges which are not Tetractinellids (see Appendix II. p. 413) we shall have to bear 
in mind that in the Tetillidse it is not so much its absence but its truthfulness when 
present that is of importance. 
The first application of this result was made in the case of the genus Samus, 
Gray ; before I had seen a specimen of this Sponge its systematic position was to me 
most perplexing, but directly its spicules were before me I recognised at once the 
characteristic sigmaspire, and my thoughts naturally reverted to the Tetillidse ; clearly, 
however, Samus cannot be placed in this family, it differs in the absence of oxeas, of 
1 Does this suggest that the sigmaspire is the final and waning term of a degenerating series of microscleres, 
commencing with an aster and proceeding downwards through the spiraster ? It would be surprising were this so, 
considering the other characters of the Tetillidse. 
