196 PROF. H. A. NICHOLSON AND DE. J. MUKIE ON THE 



in the form of rounded subhemisplierical or irregularly spheroidal 

 masses, or of flattened expansions of an incrusting character, com- 

 posed of thin, close-set laminae arranged concentrically round one 

 or more centres. These concentric and fundamentally horizontal 

 laminae are separated by interspaces, which are crossed by more 

 or less numerous vertical pillars, or, as we shall term them, 

 "radial pillars." Hence the vertical section exhibits a number 

 of approximately horizontal layers and intervening spaces, the 

 latter divided by upright pillars into a number of minute vesi- 

 cular compartments. In some instances the entire laminated 

 mass has grown, as has often been described, in successive super- 

 imposed layers round some central nucleus, such as the shell of 

 a mollusk or a coral ; but this is by no means universally the case. 



On the contrary, many examples were attached by a narrow 

 base to som.e foreign body (such as a coral), developing upwards 

 into a more or less extended expansion, the under surface of which 

 is covered by a wrinkled and imperforate epithecal membrane. 



Mode of Occuerence, Condition oe Fossiltzation, and 

 Original Constitution. 



Stromatopora and its allies are found in detached masses or 

 expansions, often of very considerable dimensions, and frequently 

 in very great numbers in particular localities. They occur most 

 commonly in limestones, associated with Corals, Brachiopods, and 

 other marine fossils; but they also occur in argillaceous sediments. 

 As to the condition of fossilization in which they occur, some 

 specimens are calcareous, others are siliceous, others are partially 

 calcareous and partially siliceous. 



As to the original constitution of the hard structures of the 

 Stromatoporoids, it has generally been assumed that the skeleton 

 was calcareous. There are, however, four views which may be 

 entertained on the subject : — 



First. It may be held that Stromatopora was originally calca- 

 reous, and that all specimens which we now find in a more or less 

 completely siliceous condition owe this to their having been more 

 or less thoroughly subjected to the familiar process of " silicifica- 

 tion." 



Secondly. It may be held that Stromatopora was primitively sili- 

 ceous, and that all calcareous specimens owe their present constitu- 

 tion to the fact that the original silica of the specimen has been 

 replaced by carbonate of lime. Mr. SoUas (Ann. & Mag. Nat. Hist. 



