fram the South Coast of Devon. 3 



This, it seems to me, may be explained in the following- 

 way, viz. : — the heavy chondroid natm'C of Tethya lyncurium^ 

 and the rapidity with which the chondroid material is pro- 

 duced (for when two or three living specimens are placed in 

 sea-water in contact, they become so firmly miitcd together in 

 twenty-four hours that force is required to tear them asunder), 

 might cause the Tethya, when broken off from its pedicel, to 

 sink to the bottom directly, and at the same time to quickly 

 unite itself to the first fixed rock with which it might come into 

 contact, while the lighter nature of the Tethyadas proper and 

 the Geodidaj, together with their inability to unite themselves 

 so quickly to foreigTi objects, might lead to their drifting alx^ut 

 in the sea, until they render tlicmselves independent of tlieir 

 position by fortifying themselves all round with their peculiar 

 structures, and finally assume the subspherous form. 



Again, the specimens of Tethya lyncuriuni only come on 

 shore in heavy storms, when these have occurred at spring- 

 tides, and thus the waves at low water have wrenched them 

 off their pedicels ; for it is only towards dead low-water mark 

 that I have yet foui\d them growing on the rocks. They 

 therefore, from their tough chondi'oid nature, probably hold on 

 when portions of Pachymatisma give way, and thus, only 

 yielding to the heaviest gales, come on shore directly after 

 they have become separated from their attachments, even be- 

 fore they have time to sink into still water and become united 

 again to some fixed object. 



Such observations may account for the presence of the facet 

 of attachment in Tethya lyncuriuni, and for its absence in the 

 subspherous forms of the Tethyadai proper and the Geodida^. 



Lastly, I would take this opportunity of noticing that my 

 description and illustrations of T. lyncurium (Annals, I. c.) are 

 Avholly fallacious where they point out the existence of inter- 

 lobular grooves on the surface, excepf for the dead state, since, 

 in some specimens which I kept alive for a few days in sea- 

 water, the chondroid substance increased to such an extent on 

 the surface as not only to efface all the interlobidar grooves, 

 but, if any thing, to leave depressions over the centre of the 

 lobules themselves, just in the opposite position to that which 

 they have in the dried specimens. So much for describing- 

 objects of natural history in the dead state ; let us now direct 

 our attention to the description of the three pachytragous 

 sponges to which I have alluded in the living one. 



Dercitus niger, milii, n. var. PI. IV. fig. 1. 



Massive, spreading, fixed, variable in thickness, following 

 the sinuosities of the rock on which it grows ; compact, hard, 



1* 



