Very abundant remains, and several species, of the extinct genus have been subsequently discovered: but always in the oolitic and liassic formations of the secondary series of rocks.
The oolitic group of rocks are very rich in remains of both plants and animals: many reptiles of genera and species distinct from those here restored have been recognised and determined by portions of the skeleton. Extremely numerous are the remains of fishes, chiefly of an almost extinct order (Ganoidei), characterised by hard, shining, enamelled scales. But the most remarkable fossils are those which indisputably prove the existence, during the period of the “Great” or “Lower Oolite,” of insectivorous and marsupial mammalia—i.e., of warm-blood quadrupeds, which, like the shrew or hedgehog, fed on insects, and, like the opossum, had a pouch for the transport of the young. The lower jaw of one of these earliest known examples of the mammalian class, found in the Stonesfield slate, near Oxford, may be seen at the British Museum, to which it was presented by J. W. Broderip, Esq., F.R.S., by whom it was described in the “Zoological Journal,” vol. iii., p. 408.
It is interesting to observe that the marsupial genera, to which the above fossil quadruped, called Phascolotherium, was most nearly allied, are now confined to New South Wales and Van Diemen’s Land; since it is in the Australian seas that is found the Cestracion, a cartilaginous fish which has teeth that are most like those fossil teeth called Acrodus and Psammodus, so common in the oolite. In the same Australian seas, also, near the shore, the beautiful shell-fish called Trigonia is found living, of which genus many fossil species occur in the Stonesfield slate. Moreover, the Araucarian pines are now abundant, together with ferns, in Australia, as they were in Europe in the oolitic period.
THE LIAS.
“Lias” is an English provincial name adopted in geology, and applied to a formation of limestone, marl, and petrified clay, which forms the base of the oolite, or immediately underlies that division of secondary rocks. The lias has been traced throughout a great part of Europe, forming beds of a thickness varying from 500 to 1000 feet of the above-mentioned substances, which have been gradually deposited from a sea of corresponding extent and direction. The lias abounds with marine shells of extinct species, and with remains of fishes that were clad with large and hard shining scales. Of the higher or air-breathing animals of that period, the most characteristic were the
Enaliosauria.
The creatures called Enaliosauria or Sea-lizards (from the Greek enalios, of the sea, and sauros, lizard), were vertebrate animals, or had back bones, breathed the air like land quadrupeds, but were cold-blooded, or of a low temperature, like crocodiles and other reptiles. The proof that the Enaliosaurs respired atmospheric air immediately, and did not breathe water by means of gills like fishes, is afforded by the absence of the bony framework of the gill apparatus, and by the presence, position, and structure of the air passages leading from the nostrils, and also by the bony mechanism of the capacious chest or thoracic-abdominal cavity: all of which characters have been demonstrated by their fossil skeletons. With these characters the Sea-lizards combined the presence of two pairs of limbs shaped like fins, and adapted for swimming.
The Enaliosauria offer two principal modifications of their anatomical, and especially their bony, structure, of which the two kinds grouped together under the respective names of Ichthyosaurus and Plesiosaurus are the examples.
The Ichthyosaurus.
The genus Ichthyosaurus includes many species: of which three of the best known and most remarkable have been selected for restoration to illustrate this most singular of the extinct forms of animal life.