136 Mr. H. J. Carter on Sponges from the 
the Abhandlungen, der k.-bayer. Akademie der Wiss., II. Cl., 
xiii. Bd. 1877, wherein (transl. /. c. pp. 264-6) Prof. Zittel 
goes into the question at considerable length, noticing in one 
part (‘Ann.’ /.c. p. 264) the occurrence of a Hexactinellid 
sponge from the White Jura of Streitberg, “half calcified, 
half siliceous.” 
The objections met with by Prof. Zittel at Jena were not 
less encountered by Mr. Sollas at the Geological Society of 
London, where it appears, from the discussion that followed 
the reading of his paper, that the President ‘ thought it was 
more probable that the sponge described was one of the Calci- 
spongie”’ (/. c. p. 255). 
But putting aside the fact that a siliceous spiculemay become 
converted during fossilization into a calcareous one, there can 
be no harm in showing how improbable it is that Pharetro- 
spongia should have been a calcareous sponge, even if the latter 
ever become fossilized. 
In the first place, as regards size, the Calcispongie of the 
present day are not only all very small, but for the most part 
absolutely diminutive. Secondly, with the exception of half 
a dozen species (all that appear to be known), none are with- 
out the tri- or quadriradiate spicule ; while the acerate spicule in 
all is straight, although sometimes undulating in its course, and 
more or less spined—never, to my knowledge, simply curved 
in the form of an arc, as in the siliceous spicules of the Reni- 
erida, of which Pharetrospongia was one. ‘Thirdly, the Calci- 
spongiz are so perishable that, although growing exuberantly 
when alive for the most part on the rocks of the sea-shore, 
where they are incessantly exposed to the action of the waves, 
they here become as diffluent as Infusoria immediately after 
death—that is, at once become disintegrated, from the want 
of that horny fibre and siliceous element which makes the 
other sponges so lasting, Fourthly, and lastly, their spicules, 
whether mounted in balsam or drawn in among the foreign 
bodies forming the core of the horny fibre in the Psammone- 
mata, break up rapidly, and in a very short time, passing 
into aqueous globules, leave not “a trace behind.” Hence 
I now never mount a specimen of a calcareous sponge for 
preservation in any thing but a dry and simple cell. 
Thus size of sponge, form of spicule, perishable nature both 
of entire sponge and individual spicule make it almost impos- 
sible that Pharetrospongia and the like could ever have been 
Calcispongiz, even if we had not the proofs above stated that 
a siliceous spicule may during fossilization become a calcare- 
ous one. 
I have premised a short account of the discovery of this 
