of the Piedmontese Coast. 167 



limis animalibus incoluntur." The same remark will apply to 

 the " animals " or soft parts of the genus Helix and many others. 

 Yet the " shells " or hard parts of the Mollusca are as readily 

 distinguishable from eaeh othei-, and form as good criteria of 

 specific distinction, as the shell of a tortoise, a crab, or an insect, 

 without reference to the other parts of the animal. Were it not 

 for this test, fossil shells could no longer be regarded by the 

 geologist as " medals of creation," and the important deduc- 

 tions which have been founded on them would be nugatory and 

 valueless. 



With respect to the separation and discrimination of "va- 

 rieties," by which term naturalists understand a modification of 

 the size, colour, and api)endages of 8])ecies, and sometimes even 

 of their form, 1 have generally observed that when a difference 

 of form exists between individuals evidently belonging to the 

 same genus, in the same locality, and having the same food and 

 other conditions of habitability, it is probable that such indivi- 

 duals belong to diflferent species; but that when such difference 

 exists between individuals belonging to the same genus, which 

 inhabit distinct and separate localities, they ought to be regarded 

 only as varieties. This is by no means however a certain rule, 

 and it depends on the habits of the animal, the relative value of 

 characters which distinguish each genus, and many other cir- 

 cumstances. 



Now, putting aside the doubts which may be entertained by 

 naturalists as to the distinction of certain species, we have 

 positive data for ascertaining to some extent the distribution of 

 British and ^Mediterranean Testacea, in the work of Philippi on 

 the Mollusca of Naples and Sicily, and that of Forbes and 

 Hanley on the Mollusca of Great Britain and Ireland. The 

 descriptions and figures in each of these works are most accu- 

 rate ; and, whether all or many of the objects which have been 

 so described and figured are true species or merely varieties, the 

 same result is obtained, namely a comparison of the Testacea in 

 each of these districts. 



My first impression on examining the Testacea of the Gulf of 

 Genoa was, that the fauna of the Mediterranean was mixed, and 

 not peculiar to that sea. I found in it a large proportion of 

 species which were familiar to me as British, and others having a 

 more southern and even tropical habitat. This led me to inquire 

 whether the division into certain definite areas, which the late 

 Professor Forbes distinguished by the names of Boreal, Celtic, 

 Lusitanian, and Mediterranean, was well founded ; and the con- 

 clusion I have arrived at is, that such a division is arbitrary and 

 irreconcilable with facts. 



In Professor Forbes's Report (in 1850) to the British Asso- 



