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 I am aware, not a single entire 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- 

 with standing 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 difficulties 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 



