544 The American Naturalist. [June, 
objects were so completely changed to silica, that acid had no 
effect upon them; while others were so entirely composed of 
carbonate of lime as to be dissolved. Dr. Hinde describes and 
figures the spicules, and says that no less than one hundred 
and sixty forms were observed. These he classified into 
thirty-eight species of thirty-two genera. Some doubt may 
be expressed as to the validity of these genera and species, but 
that the spicules occur at all is sufficient evidence of the part 
the sponges played in the great formations, and indicates their 
abundance at certain periods in the past. 
Modern deep-sea dredgings have shown that sponges exist 
now in wonderful profusion. In the Indian Ocean, out of 
about a quart of material, no less than sixty-two species 
of sponges were described. Dr. Hinde in discussing the rela- 
tions between the habitats of modern and fossil sponges, notes 
the different depths at which various Atlantic species occur. 
Some of these are nearly related by their spicules to forms oc- 
curring in the flint nodule; and the conclusion is reached 
that the species there represented could have lived in water 
varying from 1 to 1700 feet deep. The resemblance between 
the spicules found in Dr. Hinde’s flint, and those occurring in 
nodules in Ireland, Westphalia’ and Belgium; and in strata’ 
varying from Cretaceous to Eocene Tertiary, indicate an ex- 
tensive distribution both in space and in time. 
The importance of the statement of Sollas that spicules are 
being continually given off by the sponge in its process of 
growth is seen when it becomes known that thick beds of sed- 
iment are largely formed of these bodies. It at once reduces 
the number of individuals which it is necessary to imagine, if 
these strata are formed of the effete products, rather than of 
the remains of individual sponges. 
Two interesting facts may be noted in conclusion, relative 
to this group of organisms. One is the great variability it 
presents. Professor Alexander Agassiz says (Three Cruises of 
the Blake, vol. 2, p. 170) that with the group all our ideas of 
species are completely upset. “It seems as if in the sponges 
we had a mass in which the different parts might be consid- 
ered as organs capable in themselves of a certain amount of 
