PREFACE. vii 



tion of particular stratigraphical horizons by means of their 

 characteristic fossils is attended with sufficient difficulties even 

 in the Tertiary and Secondary rocks ; but amongst the Palaeo- 

 zoic deposits it has been rendered largely unreliable as regards 

 all but certain particular groups of organic remains, in conse- 

 quence of the fact that it is next to impossible to be sure 

 of the identification of such a large number of typical species. 

 Thus, to take an example from the group now under consider- 

 ation, one may find Monticiilipora petropolitana, Pand., or M. 

 tuinida, Phill., or M. pulchella, E. and H., quoted in lists of 

 characteristic fossils from particular horizons in the Palaeozoic 

 rocks ; but it is not too much to say that in nineteen cases out 

 of twenty there is no guarantee that the identification of such 

 species has been based upon anything else than the almost 

 wholly worthless characters of external form and aspect. Or, 

 one finds such a form as Stcnopora fibrosa, Goldf , or Cho'tetcs 

 lycoperdon, Say, quoted as characterising some particular hori- 

 zon — the real truth being that each of these names has been 

 used to designate several wholly unrelated forms ; while it is 

 almost or quite impossible to determine what is the actual 

 species which the original author of the title had before him. 

 It is unnecessary to multiply examples, either from this par- 

 ticular group of organisms or from any other ; but I take it for 

 almost beyond dispute that the great necessity of Palseozoology 

 at present is not so much to extend its domain indefinitely by 

 the description of new species, as to ascertain the precise extent 

 of its present frontiers by the more thorough elucidation of 

 those forms which have been already named, but have not 

 been sufficiently characterised. It is upon this basis that the 

 present work has been written ; and its main object has there- 

 fore been to record the characters of a number of already 

 partially known forms, the description of new species having 

 been quite subordinated to this. No species, also, has been 



