The invertebrate fauna of the British deposits is, as might be expected, very poor until the beds of the Rhætic series are reached. In the beds below the Rhætics, the principal invertebrate remains are the tests of the crustacean genus _Estheria_, though a few obscure lamellibranch shells have been recorded. The vertebrate fauna is of great interest. A number of fishes have been found, the most remarkable of which is the genus _Ceratodus_, occurring in the Rhætic beds of Britain and lower Triassic strata of foreign countries. It is closely related to the Barramunda of the Queensland rivers belonging to the order Dipnoi. As in the Permian strata, abundance of Labyrinthodont amphibians have been discovered, and the reptiles belong to the orders Anomodontia and Rhynchocephalia. In the Rhætic beds of Britain and in still lower Triassic beds abroad the orders Ichthyopterygia and Sauropterygia (represented by _Ichthyosaurus_ and _Plesiosaurus_) are found. The Triassic rocks also yield the earliest known mammals, the best known, _Microlestes_, occurring in the Triassic rocks of Britain and the Continent. These mammals are now placed in a subclass Metatheria of the order Monotremata. The marine invertebrate fauna of the normal Triassic rocks presents some points of considerable interest. As already remarked, the fauna may be looked upon as a passage fauna between that of Palæozoic and that of Mesozoic times, the number of Palæozoic forms which pass into the Trias being approximately comparable with those which appear here and range upwards into higher Mesozoic strata. This may be well seen by examining the table given in Chapter XXI. of the Second Edition of Sir Charles Lyell's _Student's Elements of Geology_, in which three columns shew the genera of Mollusca common to older rocks, those characteristic of the Trias, and those common to newer rocks. Amongst the first are _Orthoceras_, _Bactrites_, _Loxonema_, _Murchisonia_, and _Euomphalus_, in the second column are _Ceratites_, _Halobia_ (_Daonella_), _Koninckina_, and _Myophoria_, and in the third, Ammonites, _Cerithium_, _Opis_, _Plicatula_ and _Thecidium_[98]. [Footnote 98: It has been seen that some of the Ammonites appear earlier, namely, in Permian strata. _Myophoria_ is extremely abundant in the Trias, but ranges into newer strata.] The Ammonites are largely utilised in the case of the Mesozoic strata for separation of these strata into zones, each zone being characterised by some species of Ammonite, and the researches of Mojsisovics have proved that this zonal subdivision, long adopted for Jurassic rocks, is also applicable to those of Triassic age[99]. He gives the following table of the classification of the Triassic rocks of the Mediterranean Province, which is reproduced, as it is founded upon Palæontological evidence, and will probably be widely adopted.