find in one formation, the Carboniferous Limestone, a singularly great development of one of the genera of the family, the Aviculopecten of M'Coy. The number of species of Aviculopecten taken from these rocks amounts to upwards of eighty. These with the species of the other genera of the Aviculidoe found in the Carboniferous Limestone make up a total of about 150 species of this family which have been furnished by the Mountain or Carboniferous Limestone of the British Islands. Compared with the enormous development of the Aviculidoe in Carboniferous strata, the family is very moderately represented in Jurassic and Cretaceous rocks, while Cainozoic deposits have yielded not more than one or two species. Indeed the representation of the Aviculidoe is as remarkably small in the Tertiaries as it is remarkably great in the Carboniferous rocks.
The Cyprinidoe, though ranging from Lower Silurian rocks, are not represented by a large number of species in any formation ; and the family attains its maximum development in the Jurassic system.
Though the typical genus of the Trigoniadoe is characteristic of Mesozoic strata, yet we have a genus, Lyrodesma (represented, however, by only one species, and placed by Bronn amongst the Arcadoe), carrying the range of this family down to the Caradoc rocks. The greatest number of species of Trigoniadoe have been furnished by the Inferior Oolite ; and these are all of one genus, and that the typical one of the family, Trigonia. Although the genus Trigonia has not been found in any other rocks than those of Mesozoic age, yet living Trigonioe inhabit the waters on the coasts of Australia.
One or two species of the family Anatinidoe have been found in Silurian rocks, and not more in Devonian strata ; but the Carboniferous Limestone has given us upwards of forty species. By far the greatest number of species, however, have been taken from Jurassic rocks, which have yielded upwards of one hundred and thirty species of this family. A great decrement is found in the Cretaceous system, which gives us about an equal number of species of Anatinidoe with the Tertiary deposits.
The Tellinidoe are very few in the Palaeozoic and Mesozoic rocks, and attain their maximum development in Middle Eocene strata, in which the specific forms of the typical genus Tellina are very numerous.
The estuarine and fluviatile Unionidoe, as might be expected, are not numerous in the stratified rocks of Britain, which are chiefly of marine origin. The estuarine Wealden deposits furnish us with the greatest number ; and the fluviatile beds of the Newer Pliocene yield several species of Unionidoe.
The Myadoe commence with a species of Corbula in the Middle Devonian, and have but a moderate number of species in any formation. They attain their greatest development, both of generic and specific forms, in the Tertiary strata.
Of Solenidoe, the species in any rocks older than those of Tertiary age are very rare. These later deposits, however, have given us upwards of twenty species of this family.
No fewer than six families, the Veneridoe, Pectinidoe, Lucinidoe, Os-
