the spicules, distinguishes this group so clearly from that of the 
Tetractinellidae that it is difficult to see why it should be rele- 
gated to merely sub-ordinal rank, as has been done by Prof. Sollas. 
The analogy of the Lyssakine and Dictyonine Hexactinellidae does 
not hold good with respect to the Lithistidae and Tetractinellidae ; 
for the Lithistid skeleton is not formed by the mere welding to- 
gether of the usual spicules of Tetractinellid sponges, but the spic- 
ules themselves materially differ. It is true that, not infrequently, 
detached trifid and quadrifid spicules occur in Lithistid sponges 
and show a near relation between the two orders; but in the 
same manner, simple monaxial spicules are present in all the 
orders of siliceous sponges, without being regarded as sufficient 
reason to group them all as mere sub-divisions of the Monac- 
tinellidae. | 
Prof. Sollas has stated (loc. cit. p. 393) that certain forms 
figured in Prof. Zittel’s Monograph on Coeloptychium (Taf. V figs. 
II, 12, 17) aS sponge spicules, are only casts of foramenifera. 
In this he is most certainly under a mistake, for such is the 
beautiful state of preservation of the sponge remains from the 
Westphalian Chalk, that a novice would be able to distinguish 
a sponge spicule from the cast of a foramenifer. 
