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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, 0 fossil.)
FAMILY 39.-PHYLLYRHOIDA. (1 Genus, 6 Species living, O fossil.)
FAMILY 40.--ELYSIADA. (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.)
FAMILY 42.-ATLANTIDÆ (5 Genera, 22 Species living, 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, 0 fossil.)
FAMILY 3.-CLIONIDÆ. (4 Genera, 14 Species living, 0 fossil.)
These are sedentary, bivalve, marine mollusca, having laterally symmetrical shells, but with unequal valves. Both in
Both in space and time they are the most widely distributed molluscs. They are found in all seas, and at all depths; and when any of the families or genera have a restricted range, it seems to be due to our imperfect knowledge, rather than to any real geographical limitations. In time they range back to the Cambrian formation, and seem to have had their maximum development in the Silurian period. It is not, therefore, necessary for our purpose, to do more than give the names of the families with the numbers of the genera and species, as before.
(5 Genera, 67 Species
FAMILY 1.—TEREBRATULIDÆ, living, 340 fossil.)
FAMILY 2.-SPIRIFERIDÆ. (4 Genera, 0 Species living, 380 fossil.)
FAMILY 3.-RHYNCHONELLIDÆ. (3 Genera, 4 Species living, 422 fossil.)
FAMILY 4.-ORTHIDÆ. (4 Genera, 0 Species living, 328 fossil.)
FAMILY 5.-PRODUCTIDÆ. (3 Genera, 0 Species living, 1-46 fossil.)
FAMILY 6.—CRANIADÆ. (1 Genus, 5 Species living, 37 fossil.)
FAMILY 7.-DISCINIDÆ. (2 Genera, 10 Species living, 90 fossil.)
FAMILY 8.-LINGULIDÆ. (2 Genera, 16 Species living, 99 fossil.)
The Conchifera, or ordinary Bivalve Molluscs, may be distinguished from the Brachiopoda by having their shells laterally unsymmetrical, while the valves are generally (but not always) equal. They are mostly marine, but a few inhabit fresh water. As the distribution of some of the families presents points of interest, we shall treat them in the same manner as the marine Gasteropoda.
FAMILY 1.-OSTREIDÆ. (5 Genera, 426 Species.) DISTRIBUTION.— The Ostreidæ, including the Oysters and Scallops, are found in all seas, Arctic as well as Tropical. There are nearly 1,400 species fossil, ranging back to the Carboniferous period.
FAMILY 2.-AVICULIDÆ. (3 Genera, 94 Species.) DISTRIBUTION,—The Aviculidæ, or Wing-shells and Pearl Oysters, are characteristic of Tropical and warm seas, a few only ranging into temperate regions. Nearly 700 fossil species are known from various formations ranging back to the Devonian, and Lower Silurian.
Family 3.-MYTILIDÆ. (3 Genera, 217 Species.)
DISTRIBUTION.— The Mytilidæ, or Mussels, have a world-wide distribution. There is one fresh-water species, which inhabits the Volga. There are about 350 fossil species, ranging back to the Carboniferous epoch.