Porifera. 



may rise as high as 20 per cent.« — It is improbable that sponges take the silica 

 waiited for their skelctoii froin the small qiiantity which is dissolved in the 

 sea-water. Probably they take it partly from siliceous material on the bottom of 

 the sea, »where decomposing organic matter is abundant, under the influence of 

 which alkaline sulphides are continually being formed. These sulphides may, 

 actiiig locally, decompose the clay or Silicate of alumina, setting free soluble silicic 

 acid to be absorbed and stored up by the sponges«. 



Topsent (^) of 41 species on the French Atlantic coast finds only Chondrosia 

 reniforinis not recorded from the English Channel. The voyage of the » Hiron- 

 delle« snggests, however, that near Spain greater differences exist. 



B. Porifera incalcaria. 



See Keller (1,2), Lendenfeld (^), Rauff (2), Topsent P). 



Keller (^) suggests that Placospongia does not belong to the Geodidae, but 

 comes in the neighbourhood of Spirastrella. According to the author the cortex 

 is composed of modified spirasters. »Analysirt man die Kieselkugeln näher, so 

 ergibt sich sofort, dass dieselben aus Spirastern hervorgegangen sind(c. 



Lendenfeld (2) gives a short description of some spicules of Geodia and con- 

 cludes that » die Hauptachse der Tetraxonen der einzigen Achse der Monaxonen 

 homolog ist «. 



I. Hexactinellida. 

 See Keller (1,2). 



II. Demoterellida. 

 1. Spiculispongiae. 



A. Lithistina. 

 See Keller (2), Rauff(i,3). 



B. Tetraxonina. 



See Keller (i), Lendenfeld [y], Topsent (^), Topsent {^) (a new var.). 



Topsent (^) describes Placina monolopha F. E. S. from the shores of Roseoff; 

 it was also found in the neighbourhood of Astan at a depth of 65 metres. »Elle 

 ne possede pas de cellules sph^ruleuses distinctes ; ses cellules flagellees sont grosses 

 avec un long flagellum«. A list is given of 18 species now known to be common 

 to the Channel and the Mediterranean . From Bandol, on the Mediterranean 

 coast of France, he records P. dilopha and trilopha and Corticium candelabrum. 

 P. dilopha ))Qst riche en cellules spheruleuses, incolores«. 



C. Oligosilicina. 

 See Grentzenberg, Keller {^), Topsent P,^). 



D. Pseudotetraxonina. 

 See Keller (i), Topsent p,*). 



E. Clavulina. 



See Grentzenberg, Hanitsch, Keller (i), Leidy, Topsent (^4). 



Topsent (2) gives a detailed discussion of the history and relationships of the 

 Clionidae. His final conclusions are as follows: — Family Clionidae = Clavu- 

 lidae of perforating habits. Genus (1) CUona, Grant. Spicules are tylostyles, 

 oxeas, and spirasters ; one or more of these groups may, however, be atrophied. 



