1871.] LOBLET BRITISH LAMELLIBRANCHIATA. ' 413 



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 Aviculidce 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 Bri- 

 tish Islands. Compared with the enormous development of the 

 Aviculidce in Carboniferous strata, the family is very moderately 

 represented in Jurassic and Cretaceous rocks, while Cainozoic depo- 

 sits have yielded not more than one or two species. Indeed the 

 representation of the Avicididce is as remarkably small in the Ter- 

 tiaries as it is remarkably great in the Carboniferous rocks. 



The Gyprinidce, 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 Trigoniadce is characteristic of 

 Mesozoic strata, yet we have a genus, Lyrodesma (represented, how- 

 ever, by only one species, and placed by Bronn amongst the Arcadce), 

 carrying the range of this family down to the Caradoc rocks. The 

 greatest number of species of Trigoniadce 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 foimd in any other rocks than those of Mesozoic age, 

 yet living Trigonice inhabit the waters on the coasts of Australia. 



One or two species of the family Anatinidce have been found in 

 Silurian rocks, and not more in Devonian strata ; but the Car- 

 boniferous 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 Anatinidce with the Tertiary deposits. 



The Tellinidce 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 nu- 

 merous. 



The estuarine and fluviatile Unionidce, 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 

 jdeld several species oi Unionidce. 



The Myaclce commence with a species of Corhula in the Middle 

 Devonian, and have but a moderate number of species in any forma- 

 tion. They attain their greatest development, both of generic and 

 specific forms, in the Tertiary strata. 



Of Solenidce, 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 Veneridce, Pectinidce, Lucinidce, Os- 



