550 PEOF. W. J. SOLLAS ON SOME FOSSIL SPONGES 



Mykmectum. depeessttm, sp. nov. 

 Spongites liassicus, Quenst. (?). 



Sponge small, disciform, with rounded edges, 025 inch broad, 

 Ol inch thick. Upper surface porous, radiately grooved. Canal- 

 system : central oscule absent, apparently represented by a number 

 of small circular holes or oscules around the centre of the summit ; 

 excurrent canals represented by the radiate grooves of the upper 

 surface, and by tubes which descend from the oscules obliquely into 

 the interior ; incurrent tubes descending obliquely downwards from 

 circular openings which lie on the ridges between the radiate 

 grooves, Base covered by a thin epitheca, ending in a sharp peri- 

 pheral edge against the main skeleton. Skeleton an irregular net- 

 work of fibres, from about 0-002 to 0*004 inch in diameter. 



Loc. Hampton Down. 



Hor. Great Oolite. 



This sponge is almost precisely similar, so far as one can judge 

 from macroscopic characters, to Spongiies liasicus, Quenst. (Petrefact. 

 pi. cxxxi. fig. 43, p. 343), which, as Quenstedt points out, very 

 nearly approaches S. semicinetus semiglobus, Quenst. It differs from 

 /S. rotula, the type of Zittel's genus Myrmecium, chiefly in the 

 absence of a single central oscule and excurrent cavity ; but as 

 lipogastrism is not necessarily a character of generic importance, I 

 have not thought this difference sufficient to necessitate generic 

 distinction. 



Mtemecium bieetifoeme, Quenst. Petrefact. 



A small specimen of this sponge occurs in the collection : it is 

 0*36 inch broad and .0*23 high. It appears to be almost a replica, 

 as it were, of Quenstedt's fig. 6, pi. cxxvi., representing his S. rotula 

 biretiformis. 



Genebal Obseevations. 



Mineral Characters. — All the specimens described are preserved 

 in precisely the same manner. The replacement of the siliceous 

 skeletons by carbonate of lime is most thorough and complete, so 

 that, as a rule, all fine structure is obliterated and only general form 

 remains. Thus indications of a canal in the fibres of the Hexacti- 

 nellidaa are of rare occurrence, and it is needless to add that none 

 of the minute spicules are preserved. The replacing calcite is clear, 

 transparent, and perfectly crystalline, as though it had been depo- 

 sited as an infilling of hollow casts; not milky, and markedly 

 different from the crystalline deposit of calcite in the matrix, as is 

 the case with Stauronema, where the carbonate of lime seems to 

 have been gradually substituted for the silica of the original fibre. 



Considering that the silica displaced from sponge skeletons often 

 reappears as a pseudomorph of associated calcareous structures, it 

 is curious that, if originally calcareous, the associated Pharetrones 

 should none of them show signs of siliceous replacement *. 



* At the time I wrote niy paper on Catagma, the weight of evidence appeared 



