Prof. W. Thomson on the " Vitreous " Sponoes. 117 



spicules, where these occur. I am inclined to regard the cor- 

 ticate group, as limited by Prof. Schmidt, as of ordinal value. 



Prof. Schmidt's last family are the Halisarcinae : — " Spongiee 

 molles, non fibrosa, corpuscula calcarea vel silicea non conti- 

 nentes " — equivalent to the Halisarcina of Lieberkiihn, an 

 obscure group with neither horny fibres nor siliceous spicules, 

 and consisting of little more than an extended sheet of sarcode. 



The definitions of the three orders in Dr. Bowerbank's 

 'British Spongiadee' are sufficiently simple; but I cannot 

 regard the '^ Keeatosa " as a group equivalent to the " Cal- 

 CAEEA " and the " Silicea." 



The diagnoses of the suborders are based upon some impor- 

 tant modifications in the arrangement of the spicules and horny 

 matter, which do not, however, seem to be sufficiently definite 

 for the purposes of classification. The groups are, on the 

 whole, natural. 



Admitting the value of Dr. Gray's two primary subclasses, 

 the details of his classification seem to me unsatisfactory. The 

 author divides the Siliceous Sponges into two primary groups 

 — (1) those with membranous or unarmed ovisacs, and (2) those 

 whose ovisacs are strengthened with siliceous spicules. I 

 doubt if we know enough of the nature of these peculiar 

 bodies which we for the present call ovisacs, to found upon 

 them broad distinctive character^. The present attempt to do 

 so separates to the utmost the nearly allied corticate genera 

 Tethya and Geodia, and places SpongiUa and Halichondria 

 {Isodictyon) (between which, except in the one point of the 

 structure of the " ovisacs," it is difficult to define generic dis- 

 tinctions) in different principal sections. Under the first two 

 subsections of the Malacospoe^, Dactylocalyx and Spongia 

 are associated, on account of the common character of possess- 

 ing a network, while Aphrocallistes is divorced from its beau- 

 tiful partner Euplectella. The third subsection, the Aeeno- 

 SPONGiiE, is an excellent group, apparently of ordinal value. 



Subordinate to the subsections. Dr. Gray proposes seven 

 orders and a host of genera, and very naturally anticipates the 

 general denunciation of a system which complicates the no- 

 menclature to bewilderment, and founds generic groups upon 

 such " imperfect materials" as a " bihamate spicule figured in 

 Bowerbank's ' British Sponges.' " 



The only classification which has any material advantage 

 over the older classifications of Nardo, DeBlainville, Johnston, 

 and Lieberkiihn, seems to be that of Dr. Oscar Schmidt. Du- 

 chassaing and Michelotti, Bowerbank and Gray, have each made 

 valuable individual suggestions ; but Dr. Schmidt's grouping, 

 taken as a whole, appears to be the most in accordance with 

 our knowledge of the anatomy and physiology of the class. 



