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archipelagoes. In the Philippines, for example, the proportion of the Operculata is a little more than one-seventh ;. in the Mauritius, between one-third and one-fourth; in Madeira, onefourteenth; in the whole American continent about one-eighth ; but when we come to the Antilles we find them to amount to nearly five-sixths, about half the Operculata of the globe being found there!
* Mr. Bland endeavours to ascertain the source of some of the chief genera found in the West Indian Islands, on the principle that “ each genus has had its origin where the greatest number of species is found;" and then proceeds to determine that some have had an African, some an Asiatic, and some an American origin, while others are truly indigenous. But we fear there is no such simple way of arriving at so important a result; and in the case of groups of extreme antiquity like the genera of mollusca, it would seem quite as possible that the origin of a genus is generally not where the greatest number of species are now found. For during the repeated changes of physical conditions that have everywhere occurred since the Eocene period (to go no further back) every genus must have made extensive migrations, and have often become largely developed in some other district than that in which it first appeared. As a proof of this, we not unfrequently find fossil shells where the species and even the genus now no longer exists; as Auricula, found fossil in Europe, but only living in the Malay and Pacific Islands; Anastoma and Megaspira, now peculiar to Brazil, but fossil in the Eocene of France; and Proserpina of the West Indies, found in the Eocene formation of the Isle of Wight. The only means by which the origin of a genus can satisfactorily be arrived at, is by tracing back its fossil remains step by step to an earlier form ; and this we have at present no means of doing in the case of the land-shells. Taking existing species as our guide we should certainly have imagined that the genus Equus originated in Africa or Central Asia ; but recent discoveries of numerous extinct species and of less specialized forms of the same type, seem to indicate that it originated in North America, and that the whole tribe of" horses" may be, for anything we yet know
to the contrary, recent immigrants into the Old World! This example alone must convince us, that it is impossible to form any conclusion as to the origin of a genus, from the distribution of existing species only:
The general conclusion we arrive at, therefore, is, that the causes that have led to the existing distribution of the genera and higher groups of the terrestrial mollusca are so complex, and have acted through such long periods, that most of the barriers which limit the range of other terrestrial animals do not apply to them, although the species are, in most cases, strictly limited by them. Some means of diffusion—which, though probably acting very slowly and at long intervals, and more powerfully on continents than between islands, is yet highly efficient when we consider the long duration of genera-has, to a considerable extent, dispersed them across continents, seas, and oceans. On the other hand, those mountain barriers which separate many groups of the higher vertebrates, are generally less ancient than the genera of land-shells, which are thus often distributed independently of them. In order to compare the distribution of the terrestrial mollusca on equal terms with those of land animals generally, we must take genera of the former as equivalent to family groups of the latter; and we shall, I believe, then find that the distribution of the sub-genera and smaller groups of species do accord mainly with those divisions of the earth into regions and sub-regions which we have here indicated. Mr. Harper Pease, in a communication on Polynesian Land Shells in the Proceedings of the Zoological Society for 1871 (p. 419), marks out the limits of the Polynesian sub-region, so as exactly to agree with that arrived at here from a consideration of the distribution of vertebrata ; and he says that this sub-regions (or region, as he terms it), is distinctly characterised by its landshells from all the surrounding regions. The genera (or subgenera) Partula, Pitys, Achatinella, Palaina, Omphalotropis, and many others, are either wholly confined to this sub-region or highly characteristic of it. Mr. Binney, in his Catalogue of the Air-breathing Molluscs of North America, marks out our Nearctic region (with almost identical limits) as most clearly
characterised. He also arrives at a series of sub-divisions, which generally (though not exactly) agree with the sub-regions which I have here adopted. The Palæarctic, the Ethiopian, and the Oriental regions, are also generally admitted to be well characterised by their terrestrial molluscs. There only remain the Australian and the Neotropical regions, in which some want of homogeneity is apparent, owing to the vast development and specialisation of certain groups in the islands which belong to these regions. The Antilles, on the one hand, and the Polynesian Islands, on the other, are so rich in land-shells and possess so many peculiar forms, that, judged by these alone, they must form primary instead of secondary divisions. We have, however, already pointed out the inconvenience of any such partial systems of zoological geography, and the causes have been sufficiently indicated which have, in the case of land-shells as of insects, produced certain special features of distribution.
We therefore venture to hope, that.conchologists will give us the advantage of their more full and accurate knowledge both of the classification and distribution of this interesting group of animals, not to map out new sets of regions for themselves, but to show what kind of barriers have been most efficient in limiting the range of species, and how their distribution is actually effected, so as to be able to explain whatever discrepancies exist between the actual distribution of land-shells and that of the higher animals.
There are ten families in this order, all of which, as far as known, are widely or universally distributed. Some of them are found fossil, ranging back to the Carboniferous epoch. They are commonly termed Sea-slugs, and have either a thin small shell or none. We shall therefore simply enumerate the families, with the number of genera and species as given by Mr. Woodward.
FAMILY 31.-TORNATELLIDÆ. (7 Genera, 62 Species living, 166 fossil.)
FAMILY 32.—BULLIDÆ. (12 Genera, 168 Species living, 88 fossil.)
FAMILY 33.-APHYSIADÆ. (8 Genera, 84 Species living, 4 fossil.)
FAMILY 34.-PLEUROBRANCHIDÆ. (7 Genera, 28 Species living, 5 fossil.)
FAMILY 35.—PHYLLIDIADÆ. (4 Genera, 14 Species living, O fossil.)
FAMILY 36.—DORIDÆ. (23 Genera, 160 Species living, 0 fossil.)
FAMILY 37.—TRITONIADÆ. (9 Genera, 38 Species living, O fossil.)
FAMILY 38.—ÆOLIDÆ. (14 Genera, 101 Species living, () fossil.)
FAMILY 39.-PHYLLYRHOIDÆ. (1 Genus, 6 Species living, O fossil.)
FAMILY 40.- ELYSIADÆ. (5 Genera, 13 Species living, 0 fossil.)
These are oceanic, swimming molluscs, of a delicate texture. They are found in all warm seas, and range back to the Lower Silurian epoch. There are only two families.
FAMILY 41.-FIROLIDÆ. (2 Genera, 33 Species living, 1 fossil.)
(5 Genera, 22 Species living,
FAMILY 42.-ATLANTIDÆ 159 fossil.)
These are swimming, oceanic mollusca, inhabiting both Arctic, Temperate, and Tropical seas. The three families have each a wide distribution in all the great oceans. They range back to the Silurian period.
FAMILY 1.-HYALEIDÆ. (9 Genera, 52 Species living, 95 fossil.)
FAMILY 2.-LIMACINIDÆ. (4 Genera, 19 Species living, O fossil.)
FAMILY 3.-CLIONIDÆ. (4 Genera, 14 Species living, O fossil.)