PREFACE TO FOURTH EDITION 



FT1HE general plan of this work is to give, in each group 

 of the Invertebrata, first, a short account of its 

 general zoological features with a more detailed descrip- 

 tion of the hard parts of the animals ; secondly, its 

 classification and the characters of the important genera, 

 with remarks on the affinities of some forms ; and thirdly, 

 a description of the present distribution, and the geo- 

 logical range. The account of each genus is followed by 

 the enumeration of one or more typical species, so as to 

 guide the student in making use of a large collection. 

 The illustrations are employed mainly for the purpose 

 of explaining structure and terminology, and will not 

 enable the student to dispense with the use of specimens. 

 The list of palseontological works is intended to indicate 

 where further information may be obtained in any branch 

 of the subject ; it includes works of general interest in 

 each group, and others dealing especially with British 

 fossils. 



