10 Transactions of the Society. 



with P. patelliformis, whilst P. pileolus first comes in at the base 

 of the Upper Chalk in the zone of Holastcr planus. The well- 

 marked P. nuciformis has not been found until reaching the zone 

 of Micrastcr cor-wnguinwm. 



In the lower zones of the Chalk the examples of Porosphcera are 

 comparatively rare and generally small, the large majority not 

 exceeding 6 mm. in diameter, but in the higher zones in the South 

 of England the various species are more numerous, and there is 

 a gradual increase in size, which reaches its maximum in the zone 

 of Marsapitcs, where some forms of P. globularis are 34 mm. in 

 diameter. In the next higher zone of Actin. quadratus the speci- 

 mens are smaller, but there is a reversion to larger forms in the 

 Bel. mucronata zone. 



In the Chalk of the vicinity of Flamborough, Yorkshire, the 

 various species of Porospha'ra are uniformly of small dimensions in 

 all the zones, in marked contrast to those from the higher zones on 

 the South coast. 



IV. Mineral Character and Condition of Preservation. 



The large majority of the specimens of Povospliwra from the 

 Upper Chalk of this country consist of calcite, and their radial 

 canals and the smaller interspaces of the skeletal mesh are usually 

 filled up solid with the same mineral. In thin sections seen under 

 the Microscope, the mesh appears as light grey bands by reflected 

 light ; whilst in transmitted light the bands or fibres are marked 

 off from the calcitic matrix by numerous minute granules of a 

 darkish tint, which, unless the sections are very thin, render them 

 nearly opaque. This appearance of the mesh is very similar to 

 that in many of the Pharetron Calcisponges from the Inferior 

 Oolite, the Great Oolite, and the Coral Eag, and also in some of 

 the Tertiary Calcisponges from Australia. In no instance have 

 I seen a finely radiate fibrous micro-structure in the mesh, like 

 that which characterises Millepora and Madreporarian corals 

 generally. 



In many specimens of Porosphcera the outer portions are of 

 calcite, whilst in the interior, both the skeletal mesh and the 

 matrix consist to a varying extent of silica. This has given rise 

 to an impression that the fossils were originally of silica which 

 has, in part, been replaced by calcite. That a reverse process has 

 taken place, and that the silica is of secondary origin, is shown by 

 the fact that in the silicified portion of these specimens the mesh 

 is usually very indistinct or altogether obliterated, whilst its 

 characters are retained in the outer or calcitic portion. We find, 

 moreover, that Porosphcera is frequently associated in the same 

 beds of Chalk with undoubted siliceous hexactinellid and lithistid 

 sponges, in which the skeletal mesh has been either removed, 



