ON BRITISH MARINE ZOOLOGY, 255 
the natural inference from such statement was that such species were uni- 
versally diffused through our seas. The researches embodied in this Report, 
however, put beyond question the fact that there are marked peculiarities in 
the distribution of British marine animals, and that though there are nume- 
rous species common to the whole area, there are also numerous species 
peculiar to parts of that area. We have clear evidence of more elements 
than one contributing to the composition of our submarine population, of 
a southern element, derived from the Lusitanian provinces of the European 
seas, of a northern element introduced from the Scandinavian seas, of a Celtic 
element having its centre within our own region, of an oceanic element ma- 
nifested by the floating Gasteropoda and the Pteropoda that reach our 
shores, and of an arctic element due to causes which were in action 
before the British Isles had assumed their present conformation*. The 
following statements, founded mainly on the data contained in the prece- 
ding tables, will βουνό to illustrate the phenomena, so far as this Report is 
concerned. 
The northern and southern provinces of the western coast of Great Britain 
may be distinguished by certain Mollusca of the Littoral Zone.—-Thus, in the 
extreme south, along the shores of the English Channel, we find Truncatella 
truncatula, and there only. Trochus lineatus commences its range to the 
west of Portland Island, and is found around the coasts of Devon, Cornwall 
and the Bristol Channel, until it ceases in Cardigan Bay or a little higher 
up; a similar cessation of its diffusion taking place on the opposite shores of 
Ireland. Acmea testudinalis, on the other hand, appears in the Orkneys 
(its presence in the Zetlands is doubtful), and ranges through the Hebrides 
and the Clyde region until it reaches the northern shores of Ireland and the 
northern coast of the Isle of Man; but it is not found on coasts southwards 
of those points. Chiton marmoreus ceases sooner; Littorina petrea is 
abundant in the British Channel, and equally plentiful in the Hebrides, but 
rare in the central part of the Irish sea. All the other Littorine, Chiton 
marginatus, Rissoa parva and cingillus, Patella vulgata, Trochus cinerarius, 
‘Purpura lapillus, Skenea planorbis, Mytilus edulis and Kellia rubra, are 
common throughout the area, even as they are all round the shores of the 
British Isles. Trochus umbilicatus is equally abundant throughout the area, — 
whilst on the other hand it is entirely absent from the eastern coast of 
Britain. ᾿ 
The differences between the northern and southern provinces are equally 
shown by the sublittoral testacea.—These are evident,—Ist, in the presence of 
a number of species in the south which are not found in the north, and vice 
versd ; and 2nd, in the greater frequency of the individuals and localities of 
certain species as we proceed from south to north, and vice versd ; thus— 
1. The following testacea are confined to the extreme south; they are all 
Spanish or Mediterranean species :— 
Trochus striatus. Pholas parva. 
Trochus exiguus. Ervilia castanea. 
Chemnitzia fenestrata. Cardium rusticum, 
Volva patula. Crenella rhombea. 
Pholadidea papyracea. 
2. The following species are peculiarly southern, but more general than 
the former; they. are also species of the Mediterranean and Lusitanian 
type :— 
* See the Memoir on the British Fauna and Flora, in the first volume of the Memoirs of 
the Geological Survey. 
