75 



On account of the detached and free condition, in which 

 most of the sponge spicules here described, are met with, it 

 is apparent that nothing more than an approximate estimate 

 can be formed of the number of species and genera which 

 are represented in the contents of this flint. A certain degree 

 of accuracy can be obtained in the case of many Lithistid 

 and Hexactinellid sponges in which the peculiar form of the 

 individual spicules and the not seldom instances in which 

 these remain together, mark off one species from another; 

 but in the case of Monactinellid and Tetractinellid sponges in 

 which only isolated spicules are met with, and are often 

 closely alike in different species, there is great difficulty in de- 

 ciding to how many species the various spicules may belong. 

 Making due allowance for this fact, I estimate that the 1 60 

 different forms of spicules which are here described and figu- 

 red from the cavity of this one chalk-flint belong to 38 spe- 

 cies and 32 genera of sponges. These are divided into 4 

 species of 3 genera of Monactinellid sponges; 2O species of 

 7 genera of Tetractinellids; 6 species of 5 genera of Lithis 

 tids ; and 8 species of 7 genera of Hexactinellids. Whatever 

 may be the opinion as to the validity of so many species 

 and genera, no doubt can remain that these spicules and frag- 

 ments of sponges which are thus mingled together in one lo- 

 cality and in so small a quantity of material, bear incontrovertible 

 evidence of a very great development of the different orders 



