species is also indicated by numerals ; but it is not intended that these numbers are to be regarded as other than a record of what has been already arrived at by palaeontological investigation. Further research will add much to our knowledge not only of the forms contained in each series of strata, called by the not altogether satisfactory name formation, but also of the generic and family affinities of the species at present recorded.
The names of the families are arranged in the order of their incoming or earliest appearance in British strata. We can thus see at once the relative antiquity of the various families, so far as it is indicated by the results of research confined to the British area.
From this Table it will be seen that all the genera represented in our rocks may be classed under twenty-two heads, or patronymic designations. As no species of the family Tridacnidoe has yet been discovered in British strata, that name does not appear in the table. Though not occurring in the strata of these islands, species of Tridacna are found in the Miocene deposits of Poland.
The first family mentioned in Table I., the Arcadoe, ranges from the Llandeilo rocks to the most recent, and has species living in our present seas. The maximum development of the Arcadoe was in the Jurassic epoch, though the species from any one formation of that system of rocks are not equal in number to those from the Carboniferous Limestone. A large number of species of Arcadoe, chiefly of the typical genus Area, have been taken from the Cretaceous strata, and a still larger number (upwards of seventy) from Tertiary deposits.
The Cardiidoe and Astartidoe also commence in the Llandeilo rocks, and have equal ranges with the Arcadoe, though not a similar distribution. The former of the two families, Cardiidoe, did not attain its greatest development until the Older Pliocene or Crag period, while the Inferior Oolite has hitherto yielded the largest number of species of Astartidoe, though the specific forms of this family are almost equally numerous in the Older Pliocene deposits. But though the maximum number of species of Astartidoe have been obtained from a Tertiary formation, the Jurassic system gives us a much greater number of species than all the Tertiary formations. With the exception of the Carboniferous Limestone, the Palaeozoic rocks have furnished us with few species of either Astartidoe or Cardiadoe.
Pour families, the Mytilidoe, Aviculidoe, Cyprinidoe, and Trigoniadoe, commence their ranges in Caradoc strata ; and two of the four (Mytilidoe and Aviculidoe) are represented by a considerable number of species in those rocks. In the Devonian system the Mytilidoe are sparingly represented ; but when we rise to the Carboniferous Limestone we find this family largely developed. The Jurassic rocks, however, yield the greatest number of species of Mytilidoe, upwards of seventy having been described from the Jurassic strata of Britain. A much smaller number have been found in Cretaceous and Tertiary deposits, though the typical genus of the family, Mytilus, is abundant at the present day in the form of the common mussel (Mytilus edulis).
The distribution of the Aviculidoe is very extraordinary, since we
