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The following species are common to Madeira, and Scandinavia.
,, opercularis maximus Anomia ephippium
Being of the Madeira species
Acephala lamellibranchiata...... Acephala palliobranchiata Pteropoda Gasteropoda prosobranchiata Gasteropoda opisthobranchiata Total 34 in 156, or 21 per cent. of Madeira species common to Scandinavia.
Madeira possesses, in common with the British seas—
16 in 54, or 30 per cent.
pallida exigua Phasianella pullus Rissoa crenulata
gracilis Lachesis minima
Total of Madeira species common to the British seas, 69 in 156, or 44 per cent.
All the species common to Madeira and Britain are likewise to be found in the Mediterranean and Lusitanian district, with addition of the following:
Making of Madeira species common to the Mediterranean and Peninsula;
Total 110 in 156, or 70 per cent.
All the Madeira species were obtained in the Canary Islands, except the following:-
47 in 54, or 87 per cent.
5, " 60
Cardium Suecicum, Irish Sea
Panopoa Norvegica, North Sea
Astarte elliptica, Clyde and North Sea
Consequently the Madeira species common to the Canaries are—
Total, 129 in 156, or 83 per cent.
Turritella? (Aclis ?)
From the foregoing statements it will appear that several species and forms typical of the Arctic fauna range far to the southward, whil scarcely one of those characteristic of warm latitudes extends into hig northern regions. This would appear in a still greater degree, were th more southern districts as thoroughly explored as have been the coasts of Britain, and the fact that such has not been the case should always be borne in mind when drawing a comparison between the Mollusca of Britain and of foreign countries. It will also be seen that the Acephala, animals gifted with smaller power of locomotion, are more widely distributed than the Gasteropoda.
I now proceed to give the result of my observations regarding the particular points at which certain species and forms reach the extreme limit of their range, northward or southward.
Although, as already remarked, the transition from one fauna to another is effected gradually, yet there are certain geographical points at which a considerable change is observed to take place. The following northern species reach their most southern habitat about the northern and central parts of the British seas, though a few of them re-appear on the Nymph bank, a kind of Arctic outpost off the south of Ireland.
The following are northern species, extending only to the British Channel, or but little to the south of it.
Trophon clathratus, Irish Sea
Philine quadrata, North Sea
Crenella discors, I have never met with south of the British seas, and suspect that when reported from the south of Europe, it has been confounded with Crenella marmorata, and Crenella costulata. Philippi's description evidently applies to the former.
The following find their southern limit in the neighbourhood of Vigo.
Littorina littoralis, and Littorina rudis are met with in Lisbon.
I refer to lists already given for northern species, which are to be found in the Mediterranean, and the Canaries, most of which probably do not extend far to the south of these districts. Ceratisolen legumen, Venus striatula, Patella pellucida, Acmora virginea, and Trochus cinerarius? appear to reach their extreme southward limit about Mogador.
Proceeding from the south northward, we find the following species
of tropical type to be recorded from the Canary Islands, but not to have
been met with in a more northern locality :
Marginella glabella and a few other species, probably belonging to tropical Africa, reach as far north as Mogador.
The coast from Cadiz to Cape St. Vincent appears to mark the northern limit of various members of the Mediterranean fauna. The following species, inhabitants of the south coast of Spain and Portugal, and of the Atlantic, are not recorded to have been obtained further north than Cape St. Vincent :
crenata Vermetus, all the species Natica Guillemini