Still more remote are the insects of the Lias of Gloucestershire, yet they too can be referred to well-known family types--Carabidæ, Melolonthidæ, Telephoridæ, Elateridæ, and Curculionidæ, among beetles; Gryllidæ and Blattidæ among Orthoptera; with _Libellula_, _Agrion_, _Æshna_, _Ephemera_, and some extinct genera. When we consider that almost the only vertebrata of this period were huge Saurian reptiles like the _Icthyosaurus_, _Plesiosaurus_, and _Dinosaurus_, with the flying Pterodactyles; and that the great mass of our existing genera, and even families, of fish and reptiles had almost certainly not come into existence, we see at once that types of insect-form are, proportionately, far more ancient. At this remote epoch we find the chief family types (the _genera_ of the time of Linnæus) perfectly differentiated {168}and recognisable. It is only when we go further back still, into the Palæozoic formations, that the insect forms begin to show that generalization of type which renders it impossible to classify them in any existing groups. Yet even in the coal formation of Nova Scotia and Durham, the fossil insects are said by competent entomologists to be "allied to _Ephemera_," "near _Blatta_," "near _Phasmidæ_;" and in deposits of the same age at Saarbrück near Trèves, a well-preserved wing of a grasshopper or locust has been found, as well as a beetle referred to the Scarabeidæ. More remarkable, however, is the recent discovery in the carboniferous shales of Belgium, of the clearly-defined wing of a large moth (_Breyeria borinensis_), closely resembling some of the Saturniidæ; so that we have now all the chief orders of Insects--including those supposed to be the most highly developed and the most recent--well represented at this very remote epoch. Even the oldest insects, from the Devonian rocks of North America, can mostly be classed as Neuroptera or Myriapoda, but appear to form new families. We may consider it, therefore, as proved, that many of the larger and more important genera of insects date back to the beginning of the Tertiary period, or perhaps beyond it; but the family types are far older, and must have been differentiated very early in the Secondary period, while some of them perhaps go back to Palæozoic times. The great comparative antiquity of the _genera_ is however the important fact for us, and we shall have occasion often to refer to it, in endeavouring to ascertain the true bearing of the facts of insect distribution, as elucidating or invalidating the conclusions arrived at from a study of the distribution of the higher animals. ANTIQUITY OF THE GENERA OF LAND AND FRESH-WATER SHELLS.