M. K. A. Zittel on Fossil Calcispongice. 367 
parent circles*. From this it appears that the spicules 
possess a cylindrical form. Their length varies between 008 
and OT millim., and therefore remains within very modest 
dimensions. In general the bacillar spicules of the fibres are 
of almost exactly the same size and form. In a specimen of 
Peronella multidigitata from the Greensand of Le Mans, the 
bacillar spicules lie separately in a homogeneous light-coloured 
mass, from which they are very clearly distinguished; in 
certain parts they become sparse, whilst in others the whole 
fibre, as in the first case, appears to be composed of spicules. 
The occurrence here of a few minute triradiates is worthy of 
notice. Such triradiates, scattered among bacillar spicules, 
occur more or less frequently in several genera. They differ 
considerably in their dimensions, and sometimes attain a con¬ 
siderable size. Their arms are either straight or somewhat 
curved, but never forked at the ends. It is a comparatively 
rare occurrence to find sponges in which the fibres consist ex¬ 
clusively of triradiates. A remarkable example of this kind 
is presented by Peronella cylindrica of the Upper Jura. In 
this the individual triradiates (with which quadriradiates also 
appear to be intermixed) may be clearly distinguished in well- 
preserved specimens, especially at the periphery where indi¬ 
vidual spicules have been somewhat separated and project 
with one or two of the arms from the fibre. 
I have been unable to detect axial canals either in the 
bacillar spicules or in the triradiates. 
It is not often that the spicules are to be seen so distinctly 
as in the preparations just described. Frequently an incipient 
crystallization has effaced their contours and form, and the 
skeletal fibres show an indistinctly lamellar structure, or ap¬ 
pear as if composed of prismatic corpuscles of calc spar, which 
sometimes completely coalesce with one another. In the 
sponges of the North-German Hils and the Triassic marl of 
►8t. Oassian, this state of preservation is common. 
Very frequently a total destruction of the spicules, evidently 
after the imbedding of the sponges in the strata, occurs. A 
Peronella from Le Mans has already been mentioned, in which 
the spicules lie here and there quite separate in a homogeneous 
mass. Fibres are also not unfrequently met with, of which 
one end still appears distinctly to be composed of spicules, 
whilst the other has acquired a perfectly dense constitution. 
In certain localities {e.g. near Maestricht) most of the sponges 
are characterized by structureless homogeneous fibres. 
A further alteration is effected by recrystallization of the 
* Sollas on Pharetrospongia, Quart. Journ. Geol. Soc. vol. xxxiii. (1877) 
p. 246, pi. xi. 
