CONCLUSION. 347 



group to the uppermost bed of the Ludlow formation ; and that the sequence of forms 

 seems to me to be gradual, natural, and presenting no other difference than what would 

 be expected to have taken place during so immensely extended a period as that which 

 elapsed during the deposition of the Silurian strata. 



Systems, indeed, are arbitrary combinations which may be, and undoubtedly are, 

 necessary in the present state of our knowledge, but it is evident likewise that, as our 

 geological and palgeontological acquirements become more extended and more perfected, all 

 arbitrary and provisional lines of demarcation will become gradually lessened in importance, 

 just as many of those supposed distinctions between closely alhed species very often vanish, 

 and have to be relinquished, when we are able to inspect a large series of specimens. 



The 210 "Silurian" forms enumerated in our Table are provisionally arranged into 

 twenty-eight genera ; but doubt or uncertainty may still be entertained whether some few 

 of these are good, or whether, if good genera, the species we refer to them are really 

 correctly placed, the material in our possession at present obliging us to assume more 

 than we absolutely know. That^ as well as many other difficulties, will consequently 

 have to be cleared away with time and study before we can expect to arrive at an accurate 

 knowledge of either the British or the foreign species ; and all that can be expected from 

 us at present is to do the best we can with the material at our command. 



It will be seen from the Table that it was during the Caradoc and Wenlock periods 

 that the Silurian Brachiopoda in Great Britain were specifically most abundant, and the 

 condition of the sea most suited to their development. The number of species was also 

 great during the Llandovery time; but it must always be borne in mind that these 

 estimates are strictly provisional, and represent no more than the state of our present 

 imperfect knowledge. 



NOTE. 



It is now time that this work, commenced in 1850, should come to a conclusion, and 

 I may confidently assert that each Monograph was as complete as I could make it at the 

 period of its publication. Science, however, is continually progressing ; consequently fresh 

 discoveries will demand the periodical publication of Supplements. It will, in the first 

 instance, be preferable to revise the whole subject in a new work ; and into this all fresh 

 discoveries can be recorded, but without again describing or figuring what has been 

 already sufficiently illustrated. If life and health be granted me for some few years to 

 come, I may myself attempt this new labour. It is only, however, for the Introduction and 

 the Cretaceous and Jurassic Monographs that supplementary matter has been principally 

 accumulating ; very little new, since the commencement of my researches, having been dis- 

 covered among the Palaeozoic forms, which compose fully two thirds of the entire work. 



In conclusion, I once again tender my warmest thanks to the many friends and fellow^- 

 workers who have so materially assisted me in my labours during the last twenty years. 



FINIS. 



