366 M. K. A: Zittel on Fossil Calcispongice . 
The next question is, whether the above-mentioned u Oalc.i- 
spongia fibrosa ” can be placed with the existing Calcispongise, 
or whether they belong to another section of sponges. 
The chemical constitution of the skeleton, which furnishes 
the most reliable character in the living sponges, can only be 
employed with great caution in the case of the fossil ones ; for 
not only do originally siliceous sponges occur in a calcareous 
state, but calcareous petrifactions often pass into the siliceous 
condition. It is therefore by no means unusual for one and 
the same species to occur with a siliceous and with a calca¬ 
reous skeleton. 
The microstructure of the skeleton alone is of decided im¬ 
portance for the determination of all sponges. In this respect 
the fibrous sponges present exceedingly remarkable pheno¬ 
mena, which, however, are not difficult to explain by subsequent 
chemical and physical alterations. 
According to the genera and species, the fibres vary between 
0*3 and 1 millim. in thickness; and upon this depends the 
more or less loose texture of the skeletal tissue. They are 
always irregularly curved, frequently of different strengths 
(primary and secondary fibres) in one and the same skeleton ; 
and the interspaces produced by anastomosis are of unequal 
size and always of irregular form. The old denomination, 
sponges with u vermiform skeleton,” best suits certain Calci- 
spongife with coarse irregularly crooked fibres. 
For microscopic examination thin sections alone can be 
employed, as in silicified specimens the finer structural con¬ 
ditions are destroyed. Higher powers (100-150 diameters) 
are necessary, however, than with the fossil Hexactinellidas 
and Lithistidfe, in order to obtain distinct images, as the con¬ 
stituents of the fibres are of very small size. 
If we examine a thin slice of a well-preserved Corynella 
from the Tourtia of Essen, or of a Peronella from the Green¬ 
sand of Le Mans, under a moderate power (about 50 diameters), 
the fibres appear indistinctly striated parallel to the surface. 
By the employment of higher powers the longitudinal lines 
are resolved into small bacillar spicules closely applied to one 
another, which compose the whole fibre. Sometimes they are 
clearly separated from each other by a surface-layer which 
appears dark by transmitted light; but more commonly the 
fibre appears like a light mass of calc spar in which the spicules 
are not easily recognized. Usually tire spicules are observed 
only in the longitudinal direction of the fibre, and, indeed, so 
arranged that their extremities overlie one another; so that 
their whole length is rarely visible. Quite exceptionally, thin 
transverse sections may be detected as packets of minute trans- 
