On the Action of Caustic Potash on Sponges. 285 



EXPLANATION OF PLATE VIII. 



Fig. \. Dictijna borea/is,STp.n., §: a, profile of cephalotliorax ; i, forepart 

 of caput and falces ; c, maxilla', labium, aud stermim ; d, spin- 

 ners ; e, natural size of spider. 



Fig. 2. Erigone Whymperi, sp. n., J: a, profile without legs ; b, forepart 

 of caput and falces ; c, d, e, palpus of cJ in different positions ; 

 f, genital aperture of § ; g, natural size of spider. 



Fig. 3. JErigone arctica, White : a, left palpus of c? , from the outer side ; 

 b, natural length of palpus. 



Fig. 4. Erigone psychrophila, Thor. : right palpus of c?, from the inner 

 side above. 



Fig. 5. Erigone pyi'ovocans, sp. n. : a, spider, enlarged ; b, profile without 

 legs ; c, left palpus, from beneath on the outer side ; d, natural 

 length of spider. 



Fig. 6. Erigone ve.vatrit; sp. n., § : a, spider, enlarged ; b, profile without 

 legs ; c, fore part of caput and falces ; d, genital aperture, in per- 

 spective ; e, ditto, in front ; /, natural length of spider. 



Fig. 7. Tarentula exasperans, sp.n., § : a, spider, enlarged; b, palpus and 

 palpal organs ; c, natural length of spider. 



XXXVI. — On the Changes 2iroduced in the Siliceous Skeletons 

 of certain Sponges by the Action of Caustic Potash. By W. 

 J. SOLLAS, M.A., F.G.S., formerly Scholar of St. John's 

 College, Cambridge. 



[Plate LX:.] 



Amongst the various problems which have arisen in the diffi- 

 cult study of the sponges, that as to the exact nature of the 

 skeletal network of such genera as Farrea, Dactylocalyx^ and 

 Aphrocalltstes has not been one of the most easily solved. 



Bowerbank, who was the first to express an opinion on the 

 subject, regarded the vitreo-hexactinellid network as the exact 

 representative amongst the Silicea of the horny network of the 

 kerataceous sponges. In the latter he had previously distin- 

 guished two marked types — one in which the horny fibres are 

 solid throughout {Spongia officinalis), and another in which the 

 axis of the fibre is occupied by a hollow canal ( Verongia). The 

 same difference he now stated to exist amongst the siliceous- 

 netted sponges, and upon it separated the genera DactylocalyXj 

 Iphiteon, and Myliusia, the fibres of which he regarded as 

 solid, from certain other genera {Kaliapsis, Farrea, and Puri- 

 siphonia) , which he considered to possess canaliculated or " fis- 

 tulose " fibre. The interpretation next advanced appears to 

 have originated with the late Dr. Gray, and was adopted with 

 wider application by Professor Sir Wyville Thomson *, who, 



* 'Annals and Magazine of Natural History,' February 1868, p. 114. 



