STUDIES ON THE COMPARATIVE ANATOMY OF SPONGES. 139 
socialis, the evidence placed before us does not seem to be 
conclusive as to the truly fibrous character of the skeleton, 
apart from the so-called fibres of the cortical one. 
In the two species of Sestrostomella, viz. 8S. rugosa and 
S. clavata, described by the same author, there appears, on 
the other hand, to be little doubt as to the existence of a 
truly fibrous skeleton distinct from the cortical one. The fibre 
itself, to quote the words of Dr. Hinde, “ exhibits an altogether 
different character in the form and arrangement of the com- 
ponent spicules from that which prevails in the examples of 
Verticillites and Corynella, already described.’ The 
ensuing description, however, also shows that the fibre is very 
different from that of Lelapia. ‘The central portion of the 
fibre generally appears to be occupied by a large tri- or quadri- 
radiate spicule, one ray of which extends along the central axis 
of the fibre. . . . Beyond this centrally-placed large spicule 
the remaining portion of the fibre appears to be composed of 
several different forms of triradiate spicules, whose rays are so 
compactly and intricately interlaced together that it is ex- 
tremely difficult to ascertain their complete forms with any 
degree of precision.” Amongst these smaller spicules occur 
fork-shaped triradiates somewhat similar to those of Lelapia, 
but they appear to be irregularly arranged. In view of the 
occurrence of these spicules in other recent Heteroccela, as 
already mentioned, their systematic value must be considered 
as very questionable. 
Lelapia, then, does not appear to be very closely related 
to any of the fossil Pharetrones described by Dr. Hinde. 
Whether it is more closely related to any of the other nume- 
rous fossil sponges which have been included in that group, it 
is impossible, in the absence of detailed information as to the 
form and arrangement of the spicules in the latter, to decide. 
It has, however, a truly fibrous skelelon ; and in this respect it 
agrees with the main character of the family Pharetrones as 
laid down by Zittel. We may therefore regard it as a living 
representative of the group, but whether the group itself, as it 
stands, is a natural one is another question altogether, and one 
