412 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 24, 



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 palseontological investigation. Further re- 

 search will add much to our knowledge not only of the forms con- 

 tained in each series of strata, called by the not altogether satisfac- 

 tory 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 in- 

 coming 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 Tridacnidce 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 Arcadce, ranges from the 

 Llandeilo rocks to the most recent, and has species living in our pre- 

 sent seas. The maximum development of the Arcadce 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 Carboni- 

 ferous Limestone. A large number of species of Arcadce, 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 Cccrdiklce and. Astartidce also commence in the Llandeilo rocks, 

 and have equal ranges with the Arcadce, though not a similar distribu- 

 tion. The former of the two families, Cardiidce, 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 

 Astartidce, though the specific forms of this family are almost equally 

 numerous in the Older Pliocene deposits. But though the maximum 

 number of species of Astartidce 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 Astartidce or Gardiadce. 



Four families, the Mytilidce,Aviculidce, Cyprinidce, and Trigoniadce, 

 commence their ranges in Caradoc strata ; and two of the four (Myti- 

 lidce and AvicuUdce) are represented by a considerable number of 

 species in those rocks. In the Devonian system the Mytilidce are 

 sparingly represented ; but when we rise to the Carboniferous Lime- 

 stone we find this family largely developed. The Jurassic rocks, 

 however, yield the greatest number of species of Alytilidce, upwards 

 of seventy having been described from the Jurassic strata of Britaiu. 

 A much smaller number have been found in Cretaceous and Tertiary 

 deposits, though the typical genus of the family, Mytilus, is abun- 

 dant at the present day in the form of the common mussel (Mytilus 

 edulis). 



The distribution of the AvicuUdce is very extraordinary, since we 



