extremities into wings; but this, like the want of the hind limbs in the Cetacea and Sirenia, makes such a clear distinction between them and all other mammals that, in the absence of any knowledge of any completely intermediate or transitional forms, they can be perfectly separated, and constitute as well-defined an order as any in the class. We have, however, an inkling of the mode in which the Insectivora were modified into Chiroptera shown us by the so-called Flying Lemur (_Galeopithecus_). Finally, we have the important and well-characterised group called Primates, including all the Monkeys and Man; and the question is not yet solved as to how and through what forms this is linked on to the other groups. It is commonly assumed that the Lemurs are nothing more than inferior Primates, but the interval between them in the actual fauna of the world is very great, and our knowledge of numerous extinct types recently discovered in America, said to be intermediate in characters, is not yet sufficient to enable us to form a definite opinion upon the subject. The Edentata may be taken first as standing in some respects apart from all the others; and the Primates must be placed at the head of the series. The position of the others is quite arbitrary, as none of the hitherto proposed associations of the orders into larger groups stand the test of critical investigation, and palæontological researches have already gone far to show that they are all modifications of a common heterodont, diphyodont, pentadactylate form. _Order_ EDENTATA.