Order Lithistida, Oscar Schmidt. 
In this order are comprised sponges whose skeleton is 
built up of spicules which are united together by the inter- 
locking of the extremities of their arms to form a more 
or less open mesh or net work. In addition to the spicules 
composing the mesh-work of the body of the sponge, there 
are, in most, if not all the Lithistids, other spicules, frequently 
of quite a different form to those of the mesh, which are 
arranged on the outer surface of the sponge and may be 
termed surface spicules in contradistinction to the mesh or 
body spicules. In existing sponges of this order minute ‘flesh’ 
spicules are also present, but these have not been preserved in 
the fossil examples. 
The intimate manner in which the spicules of the mesh 
are interwoven together by the curiously modified extremities 
of their arms, has enabled the sponges of this order to retain 
in many instances their form complete in a fossil condition. 
As remarked by Zittel (Studien p. 79) the spicules do not 
usually fall asunder and become detached after the death of 
the animal, and Mr. Carter has also recently stated that the 
mesh spicules of existing Lithistids are, for the most part, so 
locked together that even boiling in nitric acid does not cause 
them to detach and fall asunder. (An. Mag. N. Hist. S. 5, 
Vol. 6 p. 496). It is to this fact that the larger portion of 
known fossil sponges belong to this order, though, if one 
may judge from the proportion of the detached spicules of 
Tetractinellid sponges in this Horstead flint, as well as at 
Haldon and in Westphalia, the Tetractinellids may have been 
equally as numerous in the same strata with the Lithistids, but 
that owing to their skeleton being built up of disconnected 
spicules, and the inevitable dispersion of these in nearly all 
cases after the death of the animal, their remains have, up 
to the present, been but little noticed. At the same time 
that the skeleton of fossil Lithistid sponges has been very 
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