232 PROF. H. A. NICHOLSON AND DR. J. MURIE ON THE 



surmounted — viz., that to substantiate the latter view it is neces- 

 sary to assume as a fact a form of Sponge to the existence of which 

 living representatives do not yield evidence of an unquestionable 

 kind. This doubtless weakens the argument very considerably, 

 however much the construction of the Stromatoporee is spongiose 

 in character. Nevertheless difficulties of a kindred nature, and 

 quite as insurmountable, are to be met with, to whatever living 

 group they may be compared. 



4. To Corals Sfc. — The Stromatoporoids have been regarded by 

 various observers as referable to the Actinozoa ; and they have been 

 placed, collectively or severally, amongst the Tabulate Corals, the 

 Alcyonaria, and the Perforate Corals. As to their relationship 

 with the Tabulate Corals (such as FistuUpora), or with the Alcyo- 

 narians (such as Tubipora), it may be at once stated that their 

 minute structure, as displayed by means of thin sections, is conclu- 

 sive against their reference to either of these groups, and shows, 

 indeed, that tlieir true nature is entirely different. For the view 

 that they belong to the Zoantharia perforata more may be said. 

 There is undoubtedly a close general resemblance between some 

 of the Stromatoporoids, such as 8. discoidea, Lonsd., and ^S'. 

 granulata, Nich., and some of the Perforate Corals, such as Psam- 

 mocora and Montipora and other members of the Poritidse — this 

 resemblance depending partly upon the general similarity of the 

 reticulate calcareous skeleton of both, and partly upon the like- 

 ness of the water-canals of the former to the corallites of the 

 latter. When closely examined, however, this likeness is seen to 

 be clearly superficial, and not fundamental. The minute struc- 

 ture of the general skeleton of the Stromatoporoids differs 

 materially from that of the Perforate Corals in its arrangement ; 

 while the stellate systems of water-canals in the former, which 

 simulate corallites, are really of a totally different nature, and are, 

 under any circumstances, totally wanting in many Stromatopo- 

 roids. We may, therefore, unhesitatingly dismiss the view that 

 the Stromatoporoids are to be referred to the Corals properly so- 

 called. 



Lastly, we may compare the Stromatoporoids, briefly, with the 

 Carboniferous fossils which constitute the genus Palceacis, Haime, 

 which have recently been carefully examined by one of us along 

 with Mr. E,. Etheridge, jun., F.G.S. The species of Palceacis 

 are either free or attached to foreign bodies ; and they differ from 

 all the Stromatoporoids in the conspicuous presence of a variable 



