370 M. K. A. Zittel on Fossil Calcispongice. 
entirely of bacillar spicules. It need not surprise us, there¬ 
fore, if the skeletons of all fibrous sponges were composed 
entirely of bacillar spicules ; for it is a matter of observation 
that extinct families, or such as are richly developed in the 
older formations, are much more frequently closely allied to 
rare and isolated existing forms than to those which at the 
present day stand at the height of their development. The 
uniformity in the size and form of the spicules in the fossil 
fibrous sponges, as also the absolute deficiency of bowed 
spicules and fibulae ( Spangen ), may also be cited in favour of 
their originally calcareous nature. 
The predominance of bacillar spicules in the fossil Calci- 
spongise seems to me, however, to be of special significance 
from another point of view. According to the concurrent 
observations of Metschnikoff, F. E. Schulze, and Barrois, the 
bacillar spicules first make their appearance in the embryo of 
most Calcispongim, and only afterwards the tri- and quadri- 
radiates. Hence, according to the biogenetic fundamental 
law, the bacillar spicules are the oldest and primary skeletal 
elements, and they must therefore, even on theoretical grounds 
alone, be assumed to have been especially present in the fossil 
ancestors of the Calcispongise. 
Moreover, that the characteristic triradiates of the Calci- 
spongim are not wanting in the fossil forms has already been 
mentioned. They sometimes lie singly among the bacillar 
spicules ; sometimes, in association with quadriradiates, they 
form the whole skeleton. With a little caution any con¬ 
founding of the tri- and quadriradiates of Peronella with 
similarly formed skeletal elements of siliceous sponges ( e . g. 
Stelletta , Pachastrella , &c.) is impossible. The spicules of the 
Calcispongise, in comparison with the above-mentioned silice¬ 
ous structures, are of diminutive size ; their straight or curved 
arms are pointed or truncate, always simple , never forked; 
and their axial canals are excessively fine, in fossil forms 
generally invisible. 
Carter’s third objection relates to the small power of resis¬ 
tance of the calcareous skeletons to the force of the waves and 
chemical influences. According to the observations of that 
experienced spongologist, the recent Calcispongise, which live 
almost exclusively in shallow water, are completely destroyed 
in a very short time. In the fossil calcareous sponges the 
peculiar arrangement of the spicules in fibrous trains, as also 
the frequent presence of a delicate superficial layer, certainly 
seems to have furnished some protection against mechanical 
destruction; but that the spicules, exactly lik$ those of the 
living forms, only made a feeble resistance to chemical action, 
