vi. 
Gastropods of Europe are now mainly represented in the tropical 
seas, Those of the Pliocene age have a closer affinity to those 
living in the neighbouring seas. The Upper Pliocenes of Italy, 
for instance, contain more than 90 per cent. of the species now 
living in the Mediterranean. The influence of climate on the 
distribution of species is well marked. Arctic types of the present 
day occur in the Crags of Norfolk and Suffolk, and in the Glacial 
Pleistocene beds of northern Europe. 
CLASSIFICATION OF THE BRITISH MOLLUSCA. 
I have endeavoured in the following arrangement to give an 
outline of the genera of the Mollusca now living in Britain, or in 
the seas which wash our coasts, upon a fairly correct basis, and 
in a few instances I have revised the nomenclature, such as 
the Helicide, Trochide, Polyplacophora, and Cephalopoda. The 
main outline of the scheme is taken from Fischer's well-known 
** Manuel de Conchyliologie” ; other works have also been made use 
of. The Cephalopoda, for example, follow the ‘ Challenger Report.” 
Pelseneer has been laid under contribution for the suppression of 
the Class Pteropoda, while the “ Manual of Conchology ” has been 
followed in as far as regards the Chitons, the Helicida, and the 
Trochide, &c. In a few cases lists of the species have been given, 
In nomenclature the Jaw of priority has been kept in full view, 
and it may be noted that the word “type” is used in the strictest 
sense, as being the species intended by the author ; while “ typical 
example” implies that while it is impossible to say that that 
particular species was the type, still it, or some closely allied one, 
was intended, in so far as the describer confined himself to a 
single type-form. In the forms grouped under the genera Rissoia 
and Odostomia much remains to be done. 
