-8 3 - 



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 theni 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. 

 n, 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. 



