64 CEYLON PEARL OYSTER REPORT. 



include both tetractinellid and monaxonellid forms, they may therefore be most con- 

 veniently regarded as constituting a separate " grade." By this exclusion of the 

 Lithistida, the term Tetractinellida is confined to Sollas's group " Choristida," a 

 sense in which it has already been employed by Lendenfeld in his work on the 

 Tetractinellida of the Adriatic (13). 



Recent researches, however, have shown that the Tetractinellida, Lithistida, 

 Monaxonellida, and certain horny sponges which I propose to call Pseudoceratosa, are 

 so closely related to one another that they must certainly be included in one and the 

 same order. Indeed, it is difficult to draw a hard and fast line between these 

 sub-divisions, which merge very gradually into one another. It is certain that 

 the Monaxonellida have been derived (probably polyphyletically) from ancestral 

 Tetractinellida, and the Pseudoceratosa (or, at any rate, the great majority of them) 

 from ancestral Monaxonellida (probably polyphyletically), while the Lithistida may 

 also be of polyphyletic origin and are closely related to the Tetractinellida on the 

 one hand and to the Monaxonellida on the other. 



The sharp separation of these four groups is, therefore, to a large extent artificial, 

 but must be retained as a matter of convenience till we know a good deal more about 

 their phylogeny. It must, however, be clearly recognised that the groups Tetracti- 

 nellida, Lithistida, Monaxonellida, and Pseudoceratosa simply represent stages in a 

 complex evolutionary series, commencing with the first-named group, and that each 

 of the later stages may possibly have been reached along more than one line 

 of descent. 



Grade : TETRACTINELLIDA. 



Tetraxonida in which the primitive tetraxonid and tetractinellid condition (or a 

 possibly still more primitive triaxonid and triradiate condition) is retained by 

 some at least of the spicules, while no desmas are developed. 



Sub-order: HO MOS CLERO PH OR A. 



Tetractinellida in which microscleres and megascleres are not yet sharply differentiated 

 from one another and no triaenes are developed. 



I venture to propose the name Homosclerophora to replace the old name 

 Microsclerophora of Sollas. The latter is very misleading in that it does not 

 indicate the primitive undifferentiated chai-acter of the spicules as regards size, and 

 leaves it to be inferred that microscleres, as distinct from megascleres, are present. 



Topsent (14), followed by Minchin (12), associates the Homosclerophora (Micro- 

 sclerophora) with the " Microtriaenosa " and the " Oligosilicina " in one group, to which 

 he gives the old name " Carnosa." This appears to me a most undesirable proceeding, 

 for while the Homosclerophora (especially the Plakinidae) are evidently primitive forms, 

 as Sollas (15) long since pointed out and as is clearly shown by their anatomical 



