MINTJTE STRUCTUEE OF STEOMATOPORA AND IT3 ALLIES. 197 



ser. 4, vol, xix.) would seem to be in favour of this view, at any 

 rate in part. 



Thirdly. As has been suggested to us by Dr. W. B. Carpenter, 

 it is possible that amongst the Stromatoporce and their allies were 

 included organisms of essentially similar structure, but diflfering 

 in the fact that some of them were siliceous and others calcareous 

 (just as a similar series of morphological types can be shown to 

 have existed amongst the arenaceous and calcareous Forami- 

 nifera). 



Fourthly. It may be maintained, as was originally supposed by 

 EichwaldC LethreaEossica,' vol. i.),andas was afterwards so forcibly 

 urged by Von Eosen (' Ueber die Natur der Stromatoporen,' 1867), 

 that the skeleton of Stromatopora was originally horny, and that 

 the primitive horny fibres were replaced by silica or by carbonate 

 of lime. This view we shall not discuss further, as the researches 

 of Zittel and Soil as have completely destroyed all the collateral 

 evidence upon which Von Eosen founded his views, and which 

 was essential to the support of his argument. 



In deciding which of the above views is correct, we have three 

 kinds of evidence to consider, namely the mode of occurrence of 

 the Stromatoporoids in the beds in which they are found and 

 the condition of their associated fossils ; the microscopic appear- 

 ances of transparent sections ; and, lastly, the composition of dif- 

 ferent specimens, as shown by the effects produced by the appli- 

 cation of acids and by other modes of examination. 



The evidence under the first head is of great importance, and 

 far too little attention has hitherto been paid to it. So far as the 

 facts at present in our possession go, the general evidence is alto- 

 gether in favour of the view that Stromatopora and its immediate 

 allies were originally calcareous. The ground for this assertion 

 is the fact that (within our experience) specimens of Stromatopora 

 and its allies are always found in a calcareous condition when the 

 associated fossils are also calcareous, but that they are generally 

 more or less siliceous where the associated and primitively calca- 

 reous fossils are to any large proportion silicified. Excellent 

 examples of this are afi"orded by the occurrence of Stromatoporoids 

 in the Silurian and Devonian deposits. Thus in the Silurian 

 Limestones (such as the Trenton, Niagara, and Clinton Lime- 

 stones), in which silicification is comparatively infrequent, the 

 specimens of Stromatopora are generally calcareous, and in the 

 Magnesian Limestones of the Guelph formation (Upper Silurian) 



