vi PREFACE. 



and delusive guide. In some of my own earlier memoirs, deal- 

 ing with species of Monticulipora, I relied, as all palaeontolo- 

 gists did at that time, upon external characters, in so far as 

 these could be observed with a moderate magnifying power ; 

 and I did not recognise the absolute necessity of thin sections 

 for the proper examination of these and similar forms. The 

 result, it need hardly be said, was that the memoirs in question 

 contained various errors, with the nature and extent of which 

 few, probably, are better acquainted than I now am myself. 

 This being the case, I need only say that, with my improved 

 knowledge, I have not in any way attempted to conceal these 

 errors ; but that, on the contrary, I have used at least as much 

 freedom in correcting them as I should have done had I been 

 dealing with the work of others. 



The absolutely pressing difficulty which, in my opinion, is 

 at present the chief obstacle to the full assumption by Palseo- 

 zoology of its proper place as a truly scientific department of 

 knowledge — a vital branch of the great science of Biology 

 — is that it is almost impossible to recognise and identify so 

 many of the extinct forms of life which have been described 

 and named by the older observers. Every palseozoologist, I 

 feel sure, will, so far as his own special department is con- 

 cerned, bear me out in saying that in the Palaeozoic rocks at 

 any rate, and as regards many groups of animals, not a few 

 types bearing long current titles are either so insufficiently 

 defined that in using them one has no certainty that one 

 is really dealing with the form so named by the original 

 author of the species, or their names are applied by different 

 observers to entirely different organisms. There are few pub- 

 lic or private collections of any extent which would not afford 

 conclusive proof of the truth of the above statement. Nor is 

 it easy to exaggerate the ill effects of this condition of things 

 upon the progress of stratigraphical geology. The determina- 



