— 50 — 
often preserved entire, there seems reason to believe that in 
the majority of instances the skeleton of the sponges of this 
order after the death of the animal, fell apart and became 
resolved into its constituent spicules. Such at all events 
appears to have been the case in the Horstead Chalk. In this 
Chalk, so far as Iam aware, not a single exézve Lithistid sponge 
has been discovered, and yet in the contents of this single 
flint from the same locality there are inclosed such a number 
and variety of detached Lithistid spicules as to make it evident, 
that the order was represented by numerous species. Nor has this 
disintegration of the Lithistid sponges in the Chalk been 
brought about by mechanical influences, for the individual spicules 
are entire and present only those alterations in their structures 
which are common to all the spicules of this flint meal. Not- 
withstanding the great abundance of single spicules, there 
are but few specimens in which two or more of these spicu- 
les yet remain together in their natural connection. 
In this order there is an extraordinary variety of form 
in the individual spicules of the skeletal mesh-work, as also in 
the manner in which they are combined together. In some 
spicules, the plan of growth is similar to that of the four-armed 
spicules of Pachastrella, in others, the form is so irregular 
that no definite system of growth can be perceived. While, 
to some extent, the difticulties attending the determination of 
the affinities of the detached spicules of the Tetractinellid 
sponges are also present in the case of the Lithistids, 
yet the very peculiar form of the Lithistid spicules 
and the varied methods by which they are attached to each 
other to form the mesh of the skeleton, furnish reliable in- 
dications of their relationships; besides which the investiga- 
tions of Zittel, Carter, O. Schmidt and Sollas have supplied 
the means of comparison with a number of genera and species, 
and thus in many instances enabled the systematic position 
of the detached spicules to be satisfactorily ascertained. In 
