XXX INTRODUCTION. 



depth are in a manner linked tog-ether by species common to 

 two or more of them. Bivalve Mollusks would appear to 

 be more extensively distributed in depth, and to constitute 

 more constant links between zone and zone than univalves. 



The foreign relations of our marine Molluscan fauna may 

 be stated approximately as follows : — 



Our catalogue of Tunicata., a group to which compara- 

 tively little attention has been given, and of which our 

 account is intended to serve only as a Prodromus, includes 

 seventy-four species. There are, doubtless, many more 

 inhabiting the British seas. Out of those which we have 

 enumerated, more than one-third are unnoticed as members 

 of any extra-British fauna, and of those that have been 

 observed elsewhere, the majority is Celtic or else boreal. 

 In the main we may regard our Ascidians as immigrants 

 from more northern seas. 



The number of our indigenous Acephala^ or bivalved 

 shell-fish, may be stated at a hundred and sixty species, 

 after expunging doubtful, introduced, and spurious forms. 

 Out of this number very few indeed, not more than about 

 seven species, are known only as inhabitants of the British 

 seas, and as these are critical or very rare types, they may 

 fairly be considered as having escaped notice on foreign 

 coasts, rather than as peculiar to our own. About eighty 

 of our bivalves extend their range in the European seas 

 both northwards and southwards of our area; forty range 

 southwards into the Lusitanian province, but are not known 

 to the north of the Celtic area ; thirteen inhabit the Scan- 

 dinavian seas, but do not range to the south of Britain, 

 nor, so far as known, across the Atlantic ; twenty-seven 

 are common to the seas of Boreal and Arctic America, and 

 those of Boreal and Celtic Europe. The few remaining 

 species are such as have been only observed so far with- 



