viii INTRODUCTION. 



gathered from these details, that in undertaking the present work, the authors have neces- 

 sarily, to a great extent, entered upon an unexplored field of study, — have been compelled 

 to investigate the relations of forms which, in very many instances, have only recently been 

 brought under their notice, and respecting whose analogues some doubt or difference of 

 opinion may occasionally exist : with a sincere desire to avoid error, they have in every 

 instance rejected species of which the examples were imperfect or doubtful. 



It is with pleasure and gratitude they acknowledge the assistance which they have 

 received in the prosecution of their task, and their thanks are especially due to Professor 

 Edward Forbes, for his valuable memoir on the Echinodermata ; to D. Sharpe, Esq., for his 

 copious notes on the Nerinese, and other valuable suggestions ; to Wm. Bean, Esq., of Scar- 

 borough ; to M. Bouchard, of Boulogne ; to Professor Tennant, F.G.S. ; to S. V. Wood, Esq., 

 F.G.S. ; to Processor Buckman; to — Bravender, Esq., of Cirencester ; and to J. Bentley, 

 Esq., of Stamford, for the loan of specimens for comparison and figuring : to M. A. 

 Buvignier, of Verdun, for his little work on the ' Oolitic Fossils of the Ardennes ;'— also 

 for the opportunities afforded them in consulting the important collections of the Viscomte 

 D'Archiac; J. Baber, Esq., F.G.S.; J. S. Bowerbank, Esq., F.R.S. ; J. G. Lowe, Esq. of 

 Chippenham; Rev. P. B. Brodie, F.G.S.; E. H. Bunbury, Esq., M.P., F.G.S.; S. P. 

 Pratt, Esq., F.R.S. ; and to Professor E. Deslongchamps, of Caen, for his obliging kindness 

 in forwarding to them a suite of specimens typical of some of the species figured by him 

 in a series of memoirs, containing many valuable observations, published in the seventh and 

 eighth volumes of the ' Memoires de la Societe Linneenne de Normandie ;' as well as to 

 J. de Carle Sowerby, Esq., for the loan of many of the original specimens described in the 

 ' Mineral Conchology ;' and to G. R. Waterhouse, Esq., and S. P. Woodward, Esq., for the 

 facilities afforded to the authors in their examination of the species contained in the National 

 Collection. To the artists, Messrs. Bailey and C. R. Bone, of the Ordnance Geological 

 Survey, the authors tender their acknowledgments for the pains they have taken in the 

 general accuracy of the lithographs. 



