365 
M. K. A. Zittel on Fossil Calcispongice. 
In my first publications upon fossil sponges* I character¬ 
ized these as u Calcispongia fibrosa,” and regarded them, on 
account of the peculiar radiate microstructure of the calcareous 
fibres which frequently occurs in them, as a perfectly isolated 
extinct order. Continuing my investigations, and especially 
by the employment of higher powers, however, the fibres in 
well-preserved specimens proved sometimes to be composed of 
spiculiform bodies. This observation, which was soon after¬ 
wards confirmed by W. J. Sollas + in the case of a fibrous 
sponge consisting of carbonate of lime from the Greensand of 
Cambridge (Pharetrospongia Strahani ), led to new points of 
view, and in the first place to a comparison with the existing 
Calcispongias. 
In the latter the skeleton consists of calcareous spicules, 
of triradiate, quadriradiate, or uniaxial form, isolated, never 
amalgamated or cemented together, and usually regularly ar¬ 
ranged. The triradiates are by far of the most general oc¬ 
currence. 
According to Hackel, there are among existing Calcispongiae 
eighteen species of which the skeleton consists exclusively of 
triradiates, forty-four species which possess tri- and quadri- 
radiates, and sixty-one species which have triradiates, quadri- 
radiates, and bacillar spicules ; in eight species the skeleton 
is exclusively composed of quadriradiates; and only in six 
species are bacillar spicules alone met with. 
Hence Hackel concludes that in the Calcispongiae the tri¬ 
radiates originally and primarily play the principal part, and 
that the quadriradiates are to be regarded only as internal 
adaptive structures of the gastral surface, and the bacil¬ 
lar spicules as external adaptive structures of the dermal 
surface. 
A noteworthy peculiarity of the calcareous spicules is their 
small average size. Tri- or quadriradiates in which the long¬ 
est limb attains a length of half a millimetre are among the 
larger ones ; very often they remain considerably under this 
measurement. The bacillar spicules also are of correspondingly 
small dimensions. In opposition to most siliceous spicules, 
the axial canals of the calcareous ones are so extraordinarily 
fine that they only become visible under a very high power. 
For all further details relating to the form, structure, and 
arrangement of the skeletal elements in the living Calci- 
spongiaj I may refer to the exhaustive descriptions of Hackel 
(L c. Bd, i. pp. 170-209). 
* Zeitsehr. der deutsch. geol. Gesellsch. Bd. xxviii. p. 651: and Neues 
Jahrb. fiir Min. &c. 1877, p. 338. 
t Quart. Journ. Geol. Soc. vol. xxxiii. p. 242. 
