Tooth of _Oxyrhina_. 241. Tooth of _Carcharodon_. 242. _Andrias Scheuchzeri_. 243. Skull of _Brontotherium ingens_. 244, _Hippopotamus Sivalensis_. 245. Skull of _Sivatherium_. 246. Skull of _Deinotherium_. 247. Tooth of _Elephas planfrons_ and of _Mastodon Sivalensis_. 248. Jaw of _Pliopithecus_. 249. _Rhinoceros Etruscus_ and _R. megarhinus_. 250. Molar tooth of _Mastodon Arvernensis_. 251. Molar tooth of _Etephas meridionalis_. 252. Molar tooth of _Elephas antiquus_. 253. Skull and tooth of _Machairodus cultridens_. 254. _Pecten Islandicus_. 255. Diagram of high-level and low-level gravels. 256. Diagrammatic section of Cave. 257. _Dinornis elephantopus_. 258. Skull of _Diprotodon_. 259. Skull of _Thylacoleo_. 260. Skeleton of _Megatherium_. 261. Skeleton of _Mylodon_. 262. _Glyptodon clavipes_. 263. Skull of _Rhinoceros tichorhinus_. 264. Skeleton of _Cervus megaceros_. 265. Skull of _Bos primigenius_. 266. Skeleton of Mammoth. 267. Molar tooth of Mammoth. 268. Skull of _Ursus speloeus_. 269. Skull of _Hyoena speloea_. 270. Lower jaw of _Trogontherium Cuvieri_. PART I. PRINCIPLES OF PALÆONTOLOGY. INTRODUCTION. THE LAWS OF GEOLOGICAL ACTION. Under the general title of "Geology" are usually included at least two distinct branches of inquiry, allied to one another in the closest manner, and yet so distinct as to be largely capable of separate study. _Geology_,[1] in its strict sense, is the science which is concerned with the investigation of the materials which compose the earth, the methods in which those materials have been arranged, and the causes and modes of origin of these arrangements. In this limited aspect, Geology is nothing more than the Physical Geography of the past, just as Physical Geography is the Geology of to-day; and though it has to call in the aid of Physics, Astronomy, Mineralogy, Chemistry, and other allies more remote, it is in itself a perfectly distinct and individual study. One has, however, only to cross the threshold of Geology to discover that the field and scope of the science cannot be thus rigidly limited to purely physical problems. The study of the physical development of the earth throughout past ages brings us at once in contact with the forms of animal and vegetable life which peopled its surface in bygone epochs, and it is found impossible adequately to comprehend the former, unless we possess some knowledge of the latter. However great its physical advances may be, Geology remains imperfect till it is wedded with Palæontology,[2] a study which essentially belongs to the vast complex of the Biological Sciences, but at the same time has its strictly geological side. Dealing, as it does, wholly with the consideration of such living beings as do not belong exclusively to the present order of things, Palæontology is, in reality, a branch of Natural History, and may be regarded as substantially the Zoology and Botany of the past. It is the ancient life-history