REPORT ON THE HEXACTINELLIDA. 15 



tinellida, tliey described, iu additiou to Sume new species of the genera Myliusia and 

 Aulodictyon, a new genus Ilyalocaulus, and gave an explanation of the hollow octa- 

 hedral nodes which occur so abundantly in fossil Hexactinellida, but less frequently 

 in living forms. 



To the communications which Wyville Thomson ' made in 1877 on some specially 

 noteworthy Hexactinellida from the Challenger material, — such as Euplectella 

 suherea, Hyalonema toxeres, Folhpogon amadou, Lefroyella decora, and others, — • 

 I will refer at greater length when noting the literature of the genera and species in 

 question. 



An important addition to our knowledge of the Hexactinellida was made in 1880 by 

 0. Schmidt.^ He gave a description of the forms collected by Agassiz in the Gulf of 

 Mexico, and entered into a discussion of their aiiinities and other general questions, 

 without, however, undertaking an independent elaboration of the system. 



Schmidt does not deny the merit of Zittel's systematic grouping of all known fossil 

 and living species into genera, families, and orders, but while admitting its utility 

 for the practical purposes of the geologist, expresses his doubt whether Zittel's 

 system represents even approximately the natural affinities. He does not therefore 

 feel himself warranted in arranging the genera described in any completely articulated 

 system. 



In the living Dictyoniua, Schmidt sees only "representatives of the incompletely or 

 altogether unknown fossil Sponges, whose affinities will for ever remain concealed"; the 

 Lyssacina, on the other hand, are " for the most part so closely related to one another, 

 that the boundaries between the genera may be moved quite arbitrarily." The 

 Euplectellidse and Hyalonematid^ appear to him genuinely natural families, but he does 

 not give the same credit to Marshall's Holteniadse. 



It is a great pity that some of the genera established by 0. Schmidt have been 

 described from fragments but slightly characteristic, and sometimes so shortly that it is 

 difficult to obtain any sufficient conception of their characters. This is especially the 

 case with the new genera Diaretida, CyatheUa, Diplacodium, Pachaulidium, Rhahdo- 

 stauridium, and Leiobolidium. 



The three genera Farrea, Bowerbank, Eurete, Marshall, and Aulodictyon, Kent, are 

 united by Schmidt into a new genus Farrea, of which he found but one species, Farrea 

 facunda, represented by numerous and certainly very variable examples. A new genus, 

 Syringidium, Schmidt is inclined to refer to the Lefroyella decora, Wyville Thomson, 

 figured and briefly described in Wyville Thomson's Atlantic. 



Of the genus Cystispongia, hitherto only known in the fossil condition, he describes 

 a living representative as Cystispongia superstes. The genus Margaritella, Schmidt, is 

 supposed to be closely allied to Ccdo2)tychium, but it does not possess the perforated 



1 The Atlantic, 1877. '- Die Spongien des Meerbusens von Mexico, ii. \\ 33. 



