106 CEYLON PEARL OYSTER REPORT. 



phylogeny. They are evidently descended from more than one group of tetrac- 

 tinellid ancestors hy degeneration of the tetraxon megascleres, and have branched off 

 into an immense number of genera and species, the classification of which, as in the 

 case of all reduced forms, is extremely difficult. 



We have seen that the two chief tetractinellid sub-orders are distinguished mainly 

 by the form of the microscleres, the " Astrophora " possessing some form of aster and 

 the " Sigmatophora " sigmata. This same distinction, broadly speaking, runs through 

 the Monaxonellida also, and may be used as the basis of their sub-division into 

 " Astromonaxonellida " and " Sigmatomonaxonellida," names which I now propose as 

 the most appropriate for the two great groups into which the " Monaxonellida " are 

 by general consent divided, and approximately equivalent to the "Clavulina" and 

 " Halichondrina " of Vosmaer and of Ridley and Dendy, and to the " Hadromerina " 

 and "Halichondrina" of Topsent, &c. 



These two groups are sharply distinguished from one another. As far as I am 

 aware, there are only three cases on record of the occurrence of astrose microscleres 

 in association with sigmata or chelce, and all three are probably to be explained 

 as due to mixture of the spiculation of two distinct sponges a very frequent 

 occurrence.* 



We may regard these two great monaxonellid sub-orders as being descended from 

 the two corresponding tetractinellid sub-orders, though this is no doubt but a crude 

 way of looking at the problem, and it may well be that some of the forms which we 

 find most difficult to classify are descended directly from the Homosclerophora, and 

 others from the Lithistida, 



The great difficulty in following out this system of classification to its logical 

 conclusions lies in the fact that in many cases the microscleres, as well as the tetract 

 megascleres, have entirely disappeared, apparently by degeneration, and we are then 

 dependent upon the much less trustworthy guidance of other characters. Thus the 

 genera Halichondria, Reniera, Axinella, Sube rites, &c, with a very simple spicu- 

 lation composed exclusively of monaxonid megascleres, are in reality more modified 

 forms than Gellius, Sigmaxinella, Ilymedesmia, Spirastrella, and so on, which have 

 not lost their microscleres. 



Sub-Order: ASTROMONAXONELLIDA. 



Monaxonellida in which the microsclere, when present, is some form of aster. 



This sub-order is practically equivalent to the Hadromerina of Topsent, which 



* The cases in question are Schmidt's Sceptrella regalis (23), p. 58 [ride also Ridley and Dendy (1), 

 p. lxii., footnote] ; Fristkdt's DesmaceMa peachii var. stellifera (24) ; and Topsent's Hymeraphia Umreti (25) ; 

 ii.ll very doubtful cases. The so-called amphiasters (or " birotulates ") of certain Desmacidonidse (e.g., 

 Axoniderma, Iotrochota, Amphiastrella) are not true asters at all, but merely modified isochela?. [Further 

 discussion on this subject, necessitated by the appearance of Toi'.sknt'.s great work on the Sponges of the 

 Azores (62), will be found in the general remarks on the sub-order Sigmatomonaxonellida.] 



