368 M. K. A. Zittel on Fossil Calcispongice. 
skeletal fibres. Centres of crystallization are formed, from 
which fine rays issue in all directions ; and as these centres 
are situated, in great numbers, sometimes in the middle and 
sometimes near the margin of the fibres, the latter acquire an 
extremely fine spheroidally-fibrous microstructure. This state 
of preservation sometimes also occurs in specimens in which 
individual fibres still distinctly show that they are composed 
of spicules. 
In certain localites, for example near Wattheim, Muggen- 
dorf, and Ambey, in the Swabio-Franconian Jura, as also in 
the u Terrain k chailles” of Switzerland, the fibrous sponges 
occur entirely or partially in a siliceous condition, like most 
of the fossils occurring with them ; these, especially when they 
are imbedded in a calcareous rock, can be completely freed 
from the matrix by dilute muriatic acid, and then do not yield 
at all in beauty and freshness to the associated Hexactinellidse 
and Lithistidae. But if we examine their siliceous fibres under 
the microscope, no trace of spicular structure is to be dis¬ 
covered ; the siliceous mass appears turbid, and as if composed 
of minute, rough, granular, or vermiform corpuscles of no 
definite form. In contrast to the hyaline latticed meshes of 
the Hexactinellidae, or the transparent Lithistid elements 
which occur in the same localities, tire silicified skeletons of 
the fibrous sponges are at the utmost translucent, and always 
produce the impression of fissured and chemically altered 
structures. Sometimes only an outer rind of the sponge-body 
is silicified, the fibres of the interior remaining calcareous. In 
such cases I have always found the silicified fibres of the 
rough nature above described, the calcareous fibres, on the 
contrary, filled with distinct spicules. Now and then, indeed, 
the spicules appear to be able to preserve their form even after 
silicification; at least Sollas (l. c. p. 253), in treating Pharetro- 
spongia Strahani with dilute acid, obtained silicified parts, 
consisting of spicules, at the surface of calcareous fibres. I 
have never seen such examples; but I certainly know several 
Jurassic fibrous sponges and one Triassic one ( Stellispongia 
variabilis ) both in the calcareous and in the siliceous state; 
and in the latter every trace of minute structure is destroyed. 
This circumstance seems to me to furnish a satisfactory 
proof that the fibres were originally composed of calcareous 
spicules , and only subsequently became converted into silica. I 
therefore regard the fibrous sponges as true Calcispongice. 
This opinion contradicts the above-mentioned dictum of 
Hackel, that no fossil Calcispongias are known ; it is also in 
opposition to the views of Sollas and Carter, according to 
which a decidedly fibrous sponge from the Greensand of 
