Most of the above-named species are very prolific, several being gregarious. An important point respecting them is the fact, that they are all confined to the eastern coasts of the Atlantic. The application of this fact will be seen hereafter.
IV. As plentiful in our seas as the species constituting the European type are many forms, the relations of which are rather northern than southern, but their chief development within and around our area. These constitute the Celtic type. Such are, Bulla lignaria, Skenea depressa, Littorina littorea, rudis, and neritoides; several species of Lacuna, Nassa reticulata and macula, Purpura lapillus, Buccinum undatum, Fusus antiquus, Triton erinaceus, Natica monilifera, Patella pellucida, and lævis, Lottia virginea, Chiton marginatus, Pecten maximus, Pectunculus pilosus, Modiola vulgaris, Abra prismatica, Mactra solida, and subtruncata, Astarte danmoniensis, Venus cassina, Pullastra vulgaris, Pandora obtusa, Mya truncata and arenaria, Solen siliqua and ensis, and Saxicava rugosa. Several of these are common to the coasts of Europe and of North America. Such species have invariably a continued range north of Britain, and with two or three exceptions cease before they reach the Mediterranean.
V. In the British Seas are a number of species not known elsewhere, or else very rare on other coasts. These are mostly found in the Irish Sea, which appears to have been the centre from which they have radiated. They constitute a peculiarly British type: Chemnitzia fulvocincta, and some allied species; several Rissoæ and Odostomiæ, Skenea serpuloides and divisa, Trochus umbilicatus, Montacuti, and millegranus, Scalaria Turtoni and Trevelyana, Velutina otis, and Natica Montagui, may be cited as examples; and such bivalves as Pecten sinuosus and obsoletus.
VI. In Dr. Fleming's History of British Animals—a work the original merits of which have scarcely yet been done justice to—the range of a great number of spedes of British Mollusca is summarily stated as "from Devon to Zetland." Many of the forms thus noted, are rarely, if ever, found in the Irish Sea or in the German Ocean. Nevertheless, the note of range is quite true, and founded on the personal observations of Dr. Fleming in the Zetland Isles and north coast of Scotland, as compared with those of Montagu on the south coast of England. The Mollusca, and other animals alluded to, are not generally distributed throng the British Seas, and are mostiy of southern origin (or members of the South European fauna). They are not grouped in patches far apart, but have a continuous distribution, which, commencing at the south-western shores of England, is extended across the southern part of St George's Channel, is continued along the west coast of Ireland, round to the Hebridean Isles, and even northwards to the Zetland Seas. Thus, the species, among which are
