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Consequently the Madeira species common to the Canaries are—

Acephala lamellibranchiata ...... 46 in 54, or 85 per cent.

Acephala palliobranchiata.........

Pteropoda .........

Gasteropoda prosobranchiata

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1 1, 100

3

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5,

60

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73 90, 81

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Gasteropoda opisthobranchiata... 6 6.

Total, 129 in 156, or 83 per cent.

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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 the 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.

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The following are northern species, extending only to the British Channel, or but little to the south of it.

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Crenella discors, I have

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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

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neighbourhood of Vigo.

Rissoa cingillus

Natica monilifera
Velutina lævigata

Purpura lapillus

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, Acmaa 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:

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Cymba proboscidalis
Conus betulinus

testiculus

Cymba Neptuni

", porcina

Promethens

Guinaicus

papilionaceus

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 :Solecurtus strigillatus Solemya Mediterranea Psammobia costata Tellina Costo

planata

pulchella

punicea

Pectunculus pilosus, or

purpurascens Lima scabrella

Pecten gibbus

pes felis hyalinus

Spondylus goedaropus

Chiton Siculus

,, Rissoi

Siphonaria concinna
Dentalium rubescens

Natica intricata

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bicallosa

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the genus

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Crepidula

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Cardita, all the species
Ungulina

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fanulum
canalyculatus

Rissoa acuta

,, Bruguieri Mesalia sulcata

striata

Cerithium vulgatum
fuscatum

Scalaria pseudoscalaris

erenata

Vormetus, all the species

Natica Guillemini

macilenta

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Cymba melo, Pecten polymorphus, and Panopoa Aldebrandi, are met with as far as the neighbourhood of Lisbon. The fine Chiton rufus, the largest European representative of the genus, I have only obtained in Lisbon and Vigo, and am not aware of its being recorded from any other locality.

The following species range from the southward to the coasts of Gallicia and Asturias:

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Of the following species, the most northern known habitat is the south of Great Britain and the coasts of Ireland :—

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It is a fact to be noted, as probably bearing some relation to an ancient distribution of land, that the range of many species of mollusca, in proceeding northward, takes a curve to the west. Several inhabitants of the Mediterranean, such as Tellina balaustina, Circe minima, Psammobia costulata, Neæra all the species, Mangelia Lefroyii, Marginella læris, &c., touching upon the extremity of Cornwall, and extending round the west of Ireland to the Hebrides, although absent from the Channels and the north sea; also a few species being found common to

the coast of West Africa, the Canary, Madeira, and Azore Islands, which are not to be obtained in Morocco or the south of the European continent.

In treating of the distribution of mollusca, some notice should be taken of species which are considered to be local, that is, limited to a particular locality or a small area, though the number of these is constantly diminishing as we extend our knowledge.

A newly-discovered species is supposed to be confined to the spot where it was first obtained, until it re-appears in a locality where it was perhaps least expected to be met with; this more particularly happens with some of the deep water species. Many marine shells supposed to be peculiar to the Canary Islands are probably common to a large unexplored tract of the African coast-several of them I have found in Madeira.

The interesting shell Pleurotoma, or Mangelia teres, was first discovered by Professor E. Forbes on the coast of Lycia; it was next met with, a few years afterwards, in the Channel of the Minch, between the Isle of Skye and the outer Hebrides, and has since been procured pretty generally throughout the British seas. I have obtained it in various parts of the Mediterranean, in the Canary and Madeira Islands, and it is in Loven's enumeration of the shells of Scandinavia. Crenella rhombea, one of the rarest species of our seas, having been only found in three or four instances and one locality (off Weymouth), I have met with in the Bay of Gibraltar, Gulf of Tunis, and abundantly off Lancerote, one of the Canary Islands. Chemnitzia fenestrata, discovered only six years ago in Dartmouth harbour, has since been obtained from at least two other localities in the South of England; I have procured it in Vigo bay, and it has been found in the result of dredgings from Alexandria in Egypt. It would be easy to cite many similar instances, but what I have mentioned may suffice to prove how unsafe it is to conclude that a species is restricted to a peculiar locality, from the negative fact of its not having been found elsewhere.

For my part, I believe that very few, if any, marine species are confined to very small areas. With reference to the curious mollusk Bifrontia zanclaa, which was only known as a very rare fossil obtained at Messina, and no recent example even of the genus recorded to have been seen, but which I found living in considerable abundance at Madeira, it is probable that further researches will discover its existence in other parts of the Atlantic; if not, it will appear to be an exception, and that its present habitat is its last refuge and stronghold, after becoming elsewhere. extinct.

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