340 BRITISH SILURIAN BRACHIOPODA. 



discovery, as well as in credit to themselves. The great object of the Palseontographical 

 Society is to work out, in the most complete possible manner, every British fossil species ; 

 and the full elaboration of a single form is of more real value to Science than the 

 imperfect knowledge of ten times that number of half-known and ill-described species. 

 We cannot, however, pick and choose when we have to work out a series of fossils ; 

 we are obliged to do our best with the incomplete as well as with the perfect material at 

 our command ; and under these circumstances the student sees clearly where his work 

 can be most advantageously directed. 



In the accompanying Table, drawn up upon the same plan as that in ' Siluria,' 

 I have carefully noted all the Silurian species and their several varieties, of which 

 more or less complete descriptions and figures are given in this Monograph. There is 

 likewise noted their duration in time, as far as such is known ; but this part of the work 

 must be considered imperfect and essentially provisional ; the state of our knowledge on 

 such matters not being sufficiently advanced to entitle us to assume any other conclusion. 



The Table has been divided into eight columns only ; as these represent the great 

 divisions of the Silurian System in Great Britain. It would have been possible, no 

 doubt, to have subdivided each of these columns into two, three, or more parts, to 

 which local designations have been given by different Geologists ; but, although we could 

 in many cases have referred our species to one or more of these subdivisions, we are 

 evidently not yet in such an advanced state as to be able to do so correctly for the entire 

 group, and we should have felt at a loss in which of the minor subdivisions such and such 

 a species should be placed. It is therefore, I think, preferable for the present not to 

 attempt too much in a general Table, and to refer the student to the description of each 

 species for any further detail that can be offered ; and, indeed, in this (as already 

 intimated) we have followed what has been done by Murchison and Salter for Siluria ; 

 and several other Geologists have adopted a similar process. In the last-named work the 

 Table of Distribution is divided into seven columns ; but we have made slight alterations 

 in ours, namely, by adding a " Cambrian " column for the " Harlech Group,'' or " Upper 

 Longmynd beds " of Murchison, which underlie the lowest division of the " Lingula- 

 flags," and in which Mr. Hicks has recently discovered two species of Brachiopoda, 

 these being the earliest forms of the group hitherto discovered in the British Isles. I 

 have likewise devoted two columns to the " Llandovery period," as it has been proposed 

 to limit the TJpper Silurian at the base of the " Upper Llandovery '' or " May Hill 

 Sandstone," and to end the Lower Silurian with the " Lower Llandovery ,'' commencing 

 with the lowest division of the '' Lingula- flags '' (see pp. 30, 31). At the same time I 

 entirely agree with what is stated in ' Siluria ' by Sir Roderick Murchison and Mr. 

 Salter, namely, that these two Llandovery divisions form but one natural-history division ; 

 and I would go still further, by asserting that, as far as the Brachiopoda are concerned, 

 I can see but a single great natural-history division from the Longmynd or Harlech 



