INTRODUCTION. xili 
the student in drawing a right conclusion from them, so far as they have a bearing on any 
doubtful question of geological classification and nomenclature. 
Minute details, like those just noticed, involved a great amount of labour in the arrange- 
ment of the collection, as well as in the determination of the species. Such labour would 
be ill bestowed, were the details inaccurate: and some one may perhaps think that the 
geographical mistakes (above mentioned), committed within the first nine pages of this work, 
are but a bad omen for its general fidelity. Iam alone responsible for the localities; and, on 
such points, I have done my best to secure the work from error. During my friend’s labours 
in the Museum, hundreds of good single specimens have been rejected, because the labels were 
lost, or so damaged as to be of doubtful authority: and, for a like reason, the contents of 
several ponderous packing cases of fossils have been cast away as worthless, when it was found 
that the labels had perished, or the exact place from which the species were gathered had 
partly faded from my memory. In this way I have made many a stern, and I might almost 
say painful, sacrifice to the integrity of the Museum: and on this score I dare claim for it a 
credit fully equal to that of any other great collection which has been formed in England. 
Mistakes, however, there must be in a work like this, the materials of which were collected 
during a period of more than thirty years. For more than twenty years of that time there was 
no Museum for the reception and arrangement of the great additions made, every succeeding 
summer, to the University collection: and when it was at length unpacked, some parts of it had 
suffered great injury from the decomposition of pyrites*, as well as from other causes; though 
fortunately the Palseozoic portion had suffered least. A few of the cases of fossils collected 
by myself in Cumberland and Wales were sent to London for examination; and nearly all that 
were jointly collected by Mr Salter and myself in 1842 and 1843 were first unpacked in 
London and afterwards sent to Cambridge. Under such circumstances, loose labels may have 
become attached to wrong specimens, and two distinct parcels of fossils may have become 
mixed together. Causes such as these may have produced some errors in the geological 
grouping of the characteristic species, and in the localities given to them: and one 
inevitable consequence of such mistakes, should they in any instance have been made, 
would be an exaggeration of the number of species common to two distinct but contiguous 
formations. I can only say, once for all, that during the progress of this work I have 
done my best (in the way already pointed out) to secure the Museum against the conse- 
quences of such mistakes; and I confidently trust that they are very few in number. 
With the exception of the Appendix to the Second Fasciculus (which is from the 
pen of my friend and former fellow-labourer Mr Salter+) all the descriptions of the following 
* Tt may perhaps deserve remark, as a caution to collectors, that large boxes containing tertiary shells mixed with 
masses of fossiliferous clay were sent by me from Hampshire to the Museum in 1821 and 1825. It was intended that 
the masses of clay should be washed so that the minute fossils they contained might be collected on a filter. When the 
boxes were unpacked, after the erection of the Museum, the specimens they contained had become utterly worthless from 
the cause alluded to in the text. Not one of the larger shells was left entire. 
}~ There is a mistake of dates in the introductory words of Mr Salter’s Appendix. It was in the summer months of 
1842 and 1843 (and not in the summer months of 1843 and 1844) that I made a joint tour with him in North Wales. In 
