INTRODUCTION. vil 
had been available, the advantages to be derived from a study of it would have 
been doubtful, inasmuch as in the greater number of the fossil forms the 
characters on which the classification of recent species is based are not preserved 
with sufficient clearness to admit of comparison. 
The Plates which illustrate the present volume have been reproduced by the 
Collotype process by Messrs. Morgan and Kidd from negatives taken by myself. 
For various reasons it was often found impracticable to represent closely allied 
species on the same Plate. As isochromatic plates were chiefly used, the 
difference in the depth of colour of the various specimens accounts for the 
variation in intensity of the figures on some of the Plates. The scale on which 
the specimens are figured is also not constant, and depends entirely on the 
reduction necessary for each specimen or group of specimens in order to fill a 
10- by 8-inch plate. I trust that sufficient detail will be found in the figures to 
give a good idea of the habit of the specimens, and also, although necessarily to a 
less extent, of the form, angle, and variation of the corallites. 
GEORGE BROOK. 
