INTRODUCTION. 



species, recent and fossil, of the whole class. To each species 

 is added a reference to the pages and plates of my three 

 volumes, so that the Synopsis will serve as a systematic 

 index to the three : an alphabetical index to the present 

 volume is also given. In the LepadidaB, I gave an additional 

 specific character, derived from the softer parts of the 

 animal's body : in the Balanidae, these parts are more alike 

 in the different species, and 1 have found it impossible to 

 give a diagnostic character thus derived. In those cases in 

 which a Family contains but one genus, or a Genus but one 

 species, I have assigned my reasons for the institution of such 

 groups, but have given, as heretofore, only a single descrip- 

 tion in full : it would have been easy to have separated, by 

 analogy, this description into one for the species, another 

 for the genus or for the family ; but as I believe such separa- 

 tion and subordination of the characters would have been 

 largely conjectural, I have thought it best to act as I have 

 done, and give, thus saving useless repetitions, only a single 

 description, and leave it for my successors, when more 

 genera or species are known, to separate, with such certainty 

 as is ever possible, the generic from the specific characters. 



In nomenclature, I have endeavoured rigorously to 

 follow the rules of the British Association, and have never, 

 at least intentionally, broken through the great law of 

 priority. In accordance with the rules, I have rejected, 

 that is, as compulsory, all names given before the intro- 

 duction of the binomial system in 1758. I have given 

 much fewer synonyms than is usual in conchological 

 works ; for it is impossible to recognise with any approach to 

 certainty, several even of the common European forms, in 

 the short descriptions given by most authors; this holds 

 good in many cases in which figures, in appearance excellent, 

 have been added. I assert this the more confidently, from 

 having had the advantage of having gone over some of the 

 Linnean synonyms with Mr. S. Hanley. I may further 

 venture to express my conviction, that giving references to 

 works, in which there is not any original matter, or in which 

 the plates are not of a high order of excellence, is absolutely 

 injurious to the progress of natural history. 



Sessile Cirripedes, partly from being attached to surfaces 



