The different divisions generally distinguished as families in the sub-class of Marsupial animals, deserve in reality the rank of independent orders, for they differ from one another in manifold differentiations of the jaw and limbs, in much the same manner, although not so sharply, as the various orders of Placental animals. In part they perfectly agree with the latter. It is evident that adaptation to similar conditions of life has effected entirely coincident or analogous transformations of the original fundamental form in the two sub-classes of Marsupials. According to this, about eight orders of Marsupial animals may be distinguished, the one half of the main group or legion of which are herbivorous, the other half carnivorous. The oldest fossil remains of the two legions (if the previously mentioned Microlestes and the Dromatherium are not included) occur in the Jurassic strata, namely, in the slates of Stonesfield, near Oxford. The slates belong to the Bath, or the Lower Oolite formation—strata which lie directly above the Lias, the oldest Jura formation. (Compare p. 15.) It is true that the remains of Marsupials found in the slates of Stonesfield, as well as those which were found later in the Purbeck strata, consist only of lower jaws. (Compare p. 29.) But fortunately the lower jaw is just one of the most characteristic parts of the skeleton of Marsupials. For it is distinguished by a hook-shaped process of the lower corner of the jaw turning downwards and backwards, which neither occurs in Placental nor in the (still living) Cloacal animals, and from the existence of this process on the lower jaws from Stonesfield, we may infer that they belonged to Marsupials. SYSTEMATIC SURVEY OF CLOACAL AND MARSUPIAL MAMMALIA. I. _First Sub-class of Mammalia:_ _Forked or Cloacal Animals (Monotrema, or Ornithodelphia)._ Mammals with Cloaca, without Placenta, with Marsupial Bones. ------------------------------------------------------------------------------------ I. } =Primary Mammals= } Unknown extinct Mammalia from the { (Microlestes?) +Promammalia+ } Trias Period { (Dromatherium?) II. } 1. Aquatic beaked 1. Ornithorhynchida { 1. Ornithorhynchus =Beaked Animals= } animals { paradoxus +Ornithostoma+ } 2. Terrestrial 2. Echidnida { 2. Echidna hystrix } beaked animals { ------------------------------------------------------------------------------------ II. _Second Sub-class of Mammalia:_ _Pouched or Marsupial Animals (Marsupialia, or Didelphia)._