BY E. P. KALLMANN. 497 



varieties of A, plicata : and Siymaxinella ciocalyjdoides, in turn, 

 is found to provide no feature definitely justifying its separation 

 generically from the majority at least of the species at present 

 included in Biemna. Accordingly it becomes necessary to decide 

 upon what grounds, if any, the genera Tylodesma, AUantopJiora, 

 and Sigmaxiiiella admit of being retained. 



The distinction between Tylodfsma (olim Biemna) and Biemna 

 (olim Desmacella) deemed essential by Topsent(46), — to whom 

 the separation of the species of Ridley and Dendy's group Des- 

 macellinae into these two genera is due, — was with respect to 

 the mode of conformation of the skeleton, a halichondroid type 

 of skeleton being regarded by him as characteristic of the former 

 genus, a disposition of the megascleres in definite fibres as char- 

 acteristic of the latter : whether the megascleres were styli or 

 tylostyli was looked upon as of minor importance The same 

 distinction was emphasised by Lundbeck(30) in defining these 

 genera, though at the same time he attached equal value to 

 certain differences in their microscleric spiculation; other authors, 

 however, — -as Thiele(41), Dendy(8), and Hentschel(15), — seem 

 disposed, like Topsent, to regard it as fundamental. Neverthe- 

 less, a critical survey of the species concerned renders it evident 

 that the distinction is an arbitrary one, and incapable of being 

 maintained; in proof of which one need only refer to the fact 

 that in certain instances, as, for example, in the case of Biemna 

 microxa Hentschel(14), and of the so-called Biemna humilis 

 Thiele(41), the authors themselves show uncertainty as to the 

 genus to which the species ought rather to be assigned. Tf, how- 

 ever, the species with tylostyli or subtylostyli as megascleres 

 (typical of Tylodesma) be compared with those in which tylo- 

 stylote megascleres are absent (typical of Biemna), it is found 

 in the case of the former that the microscleres present frequently 

 comprise toxa in addition to sigmata, but never trichites or 

 microxea, whereas in the case of the latter, with one highly 

 questionable exception — viz., Desmacella frayilis Kieschnick(24), 

 — trichites or microxea are invariably present, but never toxa. 

 Accordingly there is excellent ground for the retention of the 

 genus Tylodesma, but its definition requires amendment. 



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