PRELIMINARY OBSERVATIONS. 15 
do not yet possess any book in which all our British Silurian Brachiopoda have been 
assembled, described, and adequately illustrated. 
_ Many are the genera into which the Silurian Brachiopoda have been distributed ; but 
have all these so-termed genera and subgenera been as yet sufficiently studied? Are all 
their characters understood, and can we locate in them with certainty all our presumed 
species? These are questions which may be fairly put, and we will endeavour to answer 
them in the sequel ; but it may at once be observed that, although the larger number of 
forms have, I think, been correctly identified, there are some of whose genus and species 
we cannot feel certain, and which will require to be left in doubt, until the discovery of 
more ample material enable the paleontologist to raise the veil under which they are 
still of necessity partially shrouded. 
It is well known to geologists and paleontologists that a large proportion of the 
Lower Silurian Brachiopoda of Great Britain, as well as of other countries, occur only in 
the condition of internal casts and of external impressions ; the shell itself, which originally 
occupied the space left free between the external and internal impressions, having totally 
disappeared ; and therefore, if by the means of softened gutta-percha we take a mould of 
the external impression as well as of the interior cast, we are able to reproduce the shell 
itself, both exterior and interior, as completely and as sharply as if we had just taken 
the valves from the sea. ‘his I have done to a very considerable extent,—indeed in every 
instance where the operation was possible; and I am consequently enabled to figure a 
large number of these Lower Silurian forms in a way that has not been hitherto attempted. 
Indeed, these internal and external casts admit in many cases the possibility of a more 
complete and satisfactory study of the species than we are at times able to effect with 
some of the Upper Silurian beautifully preserved bivalve specimens, but of which the 
interior is concealed from our view by the hard matrix filling the shell. he value, 
therefore, of gutta-percha as an auxilary in the study of Silurian Brachiopoda is 
considerable, and is destined to render still further service in the same direction. 
British Silurian Brachiopoda had, indeed, prior to the publication of the present 
work, received a considerable amount of study; but although one hundred and seventy- 
seven species are named in the most recently published catalogues, the whole subject 
required considerable revision, for a large proportion of these forms do not appear to have 
been sufficiently studied or illustrated ; while others are undoubted synonyms, and the 
identification of a few was exceedingly uncertain. The very considerable researches I have 
been compelled to make for the present Monograph will, I hope, remove doubt with 
reference to some of these, and also enable me to add to our list several new forms or 
species not hitherto recorded as British. 
Thanks to the kind assistance of many friends, I have had the great advantage of 
possessing for a time under my own roof, and of being able to study and draw my illus- 
trations from, the best and largest series of specimens ever assembled. To Sir R. Mur- 
chison, Prof. Huxley, and Mr. Etheridge, I am greatly indebted for the very kind and 
