Victorian Fofisils, Pt. V. 1 1 



that a. wax squeeze g:ives a very detailed impression of the 

 orisfinal structure. 



When the fossils are sliced vertically, that is, through the 

 outer and inner layers of the Avail, and examined microscopically, 

 it is seen that the internal portion with the vertical series of 

 pillars has been absorbed or dissolved during silicification, and, 

 the intervening space being almost entirely filled up by silica, 

 only the bases of the vertical pillars ar© just discernible in cer- 

 tain places on the cut surface. The internal structure is chiefly 

 visible here and there merely as a ferruginous streak, with 

 patches of partially destroyed spicular structure. In one speci- 

 men the form of the spicular mesh has been preserved in iron 

 oxide, which has evidently been the replacing material of the 

 original sponge-structure. 



In the case of the Queensland specimens, a, siliceous mould 

 has been formed over a pseudomorph, in peroxide of iron, of 

 the spicular layers ; whilst in the New South Wales examples 

 the siliceous (beekite) replacement is a, positive one, formed on 

 a calcitic base, which latter may have been a replacement of 

 an original siliceous structure. 



Form and Dimensions of the Sponge. — R. australis seems to 

 have been typically platter-shaped, widely expanded, with a 

 somewhat thick, slightly upturned rim. The central area waa 

 holloAved into a short funnel-shaped cavity, and the base, ex- 

 teriorly, was slightly prolonged and obtusely rounded. There 

 appears to be no evidence amongst our specimens for the shape 

 suggested by Etheridge and Dun, that is, spherical or top- 

 shaped ; the peculiar appearance of the specimen which those 

 authors figure. J^^ in which the tAvo AA'alls closely approximate, may 

 possibly be due to distortion or pressure prior to fossilisation. 



By the aid of a few more than usually complete examples 

 the general external aspect of this species of Receptaculites may 

 be fairly accurately restored (Plate IV., Fig. 7), and its 

 general dimensions ascertained. A more or less flat expanse 

 of about one-half the disc in one particular specimen gives a 

 breadth of 8 cm., and from this and other similarly derived 

 data AA'e may conclude that the sponge often exceeded 15 cm. 



1 Loc. supra cit., pi. viii., fig. 4. 



