8 THE VOYAGE OF H.M.S. CHALLENGER. 



azorica, Gray. The young forms described by Wy^Tlle Thomson in his memoir on 

 Holtenia, are in several points suggestive of Hyalonema. 



Among the numerous siliceo-Jibrous sponges, which Bowerbank* described at the same 

 time in the Proceedings of the Zoological Society, there are also many forms, such as 

 Caliapsis cidaris, Bowerbank, Dactylocalyx heteroformis, Bowerbank, Dactylocalyx 

 macandreivii, Bowerbank, Dactylocalyx prattii, Bowerbank, Dactylocalyx masoni, 

 Bowerbank, Dcoctylocalyx hoiverhankii, Johnson, Dactylocalyx polydtscus, Bowerbank, 

 which belong not to the Hexactinellida, but to the Lithistida. With regard to Bowerbank's 

 remarks on the structure of the Hexactinellida, his description of a special skin or 

 cortical layer with peculiar skeletal elements deserves to be noted. 



In addition to further descriptions of some new Hexactinellid species, — such as 

 Eurete simplicissima, Semper, Enpdectella oweni, Herklots and Marshall, Pheronema 

 annsB, Leidy, Semj^erella schultzii, Semper, &c., — protracted discussions on the systematic 

 position of Hyalonema and Euplectella, and on their mode of attachment, were continued 

 for several years. 



Some new genera and species from the Atlantic area were described in 1870 by Oscar 

 Schmidt,^ who, after very accurate study of the axial relations of the siliceous spicules, was 

 the first, as above noted, sharply to distinguish the Hexactinellida from all other Sponges, 

 and especially from the Lithistida, with which they had been hitherto more or less united. 



While the genera Lanuginella and Sympagella, which were then established by 0. 

 Schmidt, are without doubt true Hexactinellida, I cannot say the same of Placodictyum 

 cucumaria, 0. Schmidt. Preparations of the skeleton which exhibit broken plates and bent 

 knotted rods, were kindly given to me for examination by Prof. 0. Schmidt himself, and 

 from these I have been able to convince myself that by the action of acids the elements 

 are dissolved with the liberation of gas, so that they are evidently not of silica, but lime 

 carbonate. It seems probable, in fact, that we have to deal with a Holothurian — Thyone 

 — and this supposition is confirmed by the general form of the organism as figured. 



By means of microscopic examination — which had been but little applied to the study 

 of fossil Sponges — certain forms belonging to the genera Scyphia and Ventriculites were 

 indubitably referred by 0. Schmidt to the Hexactinellida. 



About the same time Saville Kent,^ who had found on the coasts of Sj^ain and 

 Portugal several little known Hexactinellida, including the new genera Asconema, Kent, 

 and Fieldingia, Kent, gave a systematic review of all the then known Hexactinellid 

 genera, and distributed them in two suborders, as follows : — 



I. CoEALLiosPONGiJS, Gray. Sponge body supported by an anastomosing or con- 

 tinuous reticulate skeleton. Reproductive gemmules entirely membranous, aspiculous. 



1 Pwc. Zool. Soc. Land., 1868, pp. 118-137; 18G9, pp. 66, 323, 389. 



2 O. Schmidt, Grundziige einer Spongienfauna des atlant. Gebietes, 1870. 



3 Monthly Micr. Journ., 1S70, p. 241. 



