72 BRITISH STROMATOPOROIDS. 



from an examination of their hard parts alone. Could we examine them in the living 

 condition, it is not impossible that we should find that the differences which 

 separate them from either Hydractinia or Millepora are greater than those which 

 separate the animal of the former of these recent genera from that of the latter. 



IV. SKETCH-CLASSIFICATION. 



In the present state of our knowledge it is probably impossible to give any 

 classification of the Stromatoporoids which can claim to have a more than provi- 

 sional value. Many forms are still imperfectly known ; while others have been 

 described from their external characters only, and cannot, therefore, be at present 

 placed in any system of classification based upon the minute structure of the 

 skeleton. Considering, however, that we can never have any positive knowledge as 

 to the nature of the soft parts in the Stromatoporoids, it is clear that the founda- 

 tion of any sound classification must be sought for in the construction of the 

 skeleton, and all modern observers will admit that a satisfactory acquaintance with 

 this can only be acquired by the help of the microscope and through the medium 

 of properly prepared thin sections. "When we have obtained a definite knowledge 

 of the minute structure of the skeleton, we can usually correlate this with certain 

 external characters, and it thus becomes possible to recognise many species of 

 Stromatoporoids by superficial peculiarities alone. Under the best of circum- 

 stances, however, there are always many specimens so badly preserved as regards 

 their superficial characters, that even a practised observer would fail to identify 

 them without the help of microscopic slides. Moreover, as has been already pointed 

 out, there are many specimens in which even the microscope ceases to be of much 

 service in their determination, owing to the fact that the internal structure of the 

 skeleton has been more or less altered during the process of fossilisation. Many 

 of the Stromatoporoids from the Wenlock Limestone of Gotland are in this condi- 

 tion, and this is occasionally the case with the specimens from the Wenlock Lime- 

 stone of Britain. I find a similar change to have affected most of the Stromato- 

 poroids which have been collected by Mrs. Robert Gray from the Silurian Rocks 

 of the Girvan area, and which she has been good enough to confide to me for 

 examination. In other cases, again, a long series of specimens may be examined, 

 and perhaps not more than one or two examples will be found in which the internal 

 structure is satisfactorily preserved. 



In view of the above-mentioned difficulties which attend the study of the 

 Stromatoporoids, and bearing in mind that there are yet various described types 



