334 DR.J.S8. BOWERBANK ON SILICEO-FIBROUS SPONGES. [May 13, 
the rigid skeleton ; but as these tissues on the exhalant surface are 
not nearly so distinct and regular in their structure as those of the 
inhalant surface, I could not find a piece that would have afforded 
a satisfactory figure, although when viewed beneath the microscope 
the nature and characters of the tissues were beyond a doubt. The 
furcated foliato-expando-ternate connecting spicula, when thus seen 
in situ, are so closely packed, and the terminations of their radii 
are so locked together, that they cannot be separated by the eye ; and 
the small acerate tension-spicula so profusely scattered on the dermal 
membrane covering their apices tends greatly to confuse the aspect 
of the tissues beneath: it is only when we have one of them 
separated, as represented by fig. 6, Pl. XXIII., that we are enabled to 
comprehend their structure. But although ineligible for figuring, 
this fragment of the expansile dermal system clearly demonstrated 
the agreement in general structure of this species with those in which 
it is more amply and clearly exhibited. 
The furcated foliato-expando-ternate connecting spicula are sin- 
gular in their form, and are very characteristic of the species. Both 
the primary and secondary ramifications of their apices are very 
much depressed ; they are very thin, and small short branches are 
projected from their edges so as greatly to increase their plane of 
support to the dermal membrane, which appears to have closely ad- 
hered to them in the living state, as I have not seen any separate spi- 
culum of this form without a portion of the dermal membrane and its 
numerous tension-spicula closely adhering to its external surface. 
The rectangulated hexradiate interstitial spicula appear to be few in 
number in the present condition of the sponge. They are small and 
slender, and the apices of the radii are slightly inclined to be clavate; 
the axial and rectangulating radii are usually of very nearly the same 
length,—a few of them only having the basal portions of the axial 
radii elongated to about twice that of a rectangulated ray. 
There are two sorts of spinulo-multifurcate hexradiate retentive 
spicula, with seven or eight spinulate radii to each termination :—one 
in which the primary radii are short, and the secondary ones pro- 
jected expansively, so as to form one great compound stellate spicu- 
lum, in which it is very difficult to separate with the eye the six sets 
of terminal spinulate radii; the other form in which the primary 
radii are longer and the terminal groups of spinulate spicula, usually 
six, rarely seven or eight in number, are projected contractedly so 
as to form six separate and very distinct groups of terminal spinulate 
spicula, as represented by fig. 7, Pl. XXIII. The first-mentioned 
form is very like that from Dactylocalyx pumiceus, represented by 
fig. 4, Pl. III., part 1, with the imaginary addition of as many more 
radii as are there represented. 
Mytivusta, Gray, Proc. Zool. Soc. 1859, p. 439. 
Skeleton siliceo-fibrous. Fibres sclid, cylindrical. Rete symmetri- 
cal, disposed in a series of crypt-like layers parallel with the external 
surface, with intervening planes of pertorated siliceous tissue. 
