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an anterior notch, so that the species of this remarkable family have changed in formand character in the successive faunse of the earth, and after having swarmed in such, abundance during the Oolitic and Cretaceous periods, have totally disappeared withthe latter from its surface. The changes which have taken place in the tetra-branehiate division of Cephalopoda are also most remarkable.
These Cephalopoda are divided by D’Orbigny into two great families; 1st, Nau-tilidse; 2nd, Ammonida:. In the first, D’Orbigny recognizes the genera Nautilus,Aganides (Clymenis), Cyrtoeeras, Lituites, Ortlioeeratites. All these appeared ingreat numbers and in very varied forms (including Phragmoceras) in the ancientfauna of the Silurian epoch; but, strange to say, the genus Nautilus alone occurs inthe Oolitic and Cretaceous . The Aganides reappear in the Tertiary, but the genusNautilus alone preserves to man a knowledge of the tetrabranchiate Cephalopoda ofancient worlds. The Nautilida: are distinguished from the Ammonidae by the straightor simply arched septa of their chambers, those of the Ammonidae being lohedor digitated, and by a central or medial siphuncle, that of the Ammonidae beingdorsal (more properly called ventral).
The family of Ammonidae contains seven genera, one of which only, namely,Goniatites, goes back so far as the carboniferous strata, of which it is a characteristicCephalopode , at once appearing and ending in them. As the Goniatites have theirsepta formed with either angular or rounded lobes, and not with lobes of the foliatedforms of the Ammonidae, they are very distinct from them, though associated in thesame family. With the Muschelkalk true Ammonites begin, though still withcomparatively simple septa, but in the Oolites they attain highly digitated or ramifiedsepta, and exhibit a number of species, these animals having been peculiarly abundantat this epoch. It is scarcely necessary to refer in detail to other animals, thoughthe genera Lima and Grypliaea are highly characteristic amongst the bivalves, andthe genus Trigonia, now existing only in Australia , occurs to confirm, as it were, theanalogies already established by the presence in the Oolitic strata of Marsupial Mammals . In the whole series of ancient creations, none is more remarkable thanthe Oolitic, and it has been called, from the peculiarly rich development of itsreptiles, the age of reptiles. The great marine Saurians, Ichthyosaurus andPlesiosaurus, have been already noticed, genera which seem to form a link betweenreptiles and fishes, and, as Professor Owen states, between reptiles and cetaceousmammals; but in addition to these, reptiles of the true crocodilian type appear atthis epoch. Some of these, Teleosaurus, Steneosaurus, Cetiosaurus, were, like theGavials, fitted to prey on fishes, having long narrow jaws armed with slender,conical, sharp-pointed, and equal teeth: they were indeed mighty monsters, thejaws of the Teleosaurus Chapmanni exhibiting at least 140 teeth, and the bodies ofsome of the species of the two first genera being 18 feet long, whilst in the thirdgenera some of the species attained an enormous bulk, the Cetiosaurus mediushaving possibly been 40 feet long, and others having almost rivalled the modernwhales in magnitude. Passing, however, from the marine, only remarking that Saurianswith true marine habits are now represented by only one species, the puny Ambly-rhynchus of the Gallipagos Islands, we find that the land was equally repletewith wondrous exhibitions of these gigantic reptiles, forming the Dinosaurians ofOwen, which are called by him Crocodile-lizards, being distinguished both from themodern terrestrial and amphibious Sauria, and from extinct marine lizards. Of thesethe most remarkable are the Megalosaurus, the Iguanodon, and the Hylaeosaurus.Professor Owen corrects the preceding estimates of the length of the Megalosaurus,which he considers excessive, and reduces it to 30 feet; but even this is sufficient toconstitute an enormous creature, more especially as it was more elevated and bulky