treidoe, Mactridoe, and Pholadidoe, commence their ranges in the Carboniferous Limestone. Of these six families, four (Veneridoe, Lucinidoe, Mactridoe, and Pholadidoe) attain their maximum development in the Tertiaries, and two (Pectinidoe and Ostreidoe) in Jurassic strata.
The small family Gastrochoenidoe, of which but few species have been found in British strata, has its earliest representative in the Lias, and its greatest number of species in Tertiary strata.
The fluviatile Cyrenidoe also range from the Lias, and, like the Gastrochoenidoe, have their maximum development in the Tertiaries.
The remaining two families, Chamidoe and Hippuritidoe, are numerically unimportant, and yet are structurally perhaps the most remarkable of all the Lamellibranchs, some of the forms of Chamidoe strongly resembling Gasteropods, and the Hippuritidoe being so abnormal in form and structure as to have been for a long time considered a separate order, the Rudistes of Lamarck. The Hippuritidoe are also of great interest, as being the only extinct family of the Lamellibranchiata as yet discovered. They are represented in the British Islands by only one species, the Hippurites, or Radiolites Mortoni of Mantell ; and this species is confined to the Upper Greensand and Chalk.
Though we are able somewhat satisfactorily to divide the Lamellibranchiata into families, there are not sufficiently good distinctions for ordinal divisions. The fact of some genera, however, having only one adductor muscle has been seized as a basis on which a division of the class into the orders, groups, or sections Monomyaria and Dimyaria has been made. This division, though perhaps not a sufficiently scientific or philosophical one, is in some respects convenient, and it has been adopted by several authors.
A better division of the Lamellibranchiata than into Monomyaria and Dimyaria is perhaps that founded on the possession, by a large number of genera, of respiratory siphons, and some being unprovided with those organs. This great structural difference naturally divides the class into Asiphonida and Siphonida — the Siphonida, or those furnished with respiratory siphons, including by far the greater number of genera. These two great sections of the class have an equally extended range ; but the Asiphonida have their greatest development in Jurassic rocks, while the greatest number of species of Siphonida have been furnished by Tertiary strata. It may be mentioned, however, with respect to the former section, that although the Jurassic system, which includes all beds from the Lower Lias to the Purbeck, yields a greater number of species of Asiphonida than any other of the great groups or systems of rocks, yet when we compare formations we find that no single formation equals the Carboniferous Limestone in its yield of species of this division of the Lamellibranchiata. (See Table II.)
The distribution of the entire class now demands our attention ; and disregarding generic, family, ordinal, or sectional divisions, and having regard only to species, we find the Lamellibranchiata sparingly represented in the lower division of the Lower Silurian group ;
