Dr. Peters, in his two papers on the Eared Seals (_Otaria_) uses the length of the ears and the existence or non-existence of the under-fur, as well as the characters used by Mr. Gill and myself, to separate the species of these animals into subgenera. The length of the ears may probably afford good characters for the separation of the species and groups, if they can be observed in the living animals. As yet, only one species of these animals, the Sea-lion or Sea-bear (_Otaria leonina_), has been observed alive in Europe; so that Dr. Peters’s notes could only be derived from the examination of more or less carefully preserved skins; and, I fear, little dependence can be placed on them. The form of the hinder opening of the nostrils and the form of its front edge, when only one or two skulls of a species were examined, have been regarded as constituting a good character; but when an extensive series of the skulls of a single species, or of several species, have been examined, that part is found to vary considerably as to the width of its different parts, and especially in the form of its front edge. As far as my observations have extended, the hinder opening of the nostrils appears to become narrower, and especially its front edge, as the animal becomes adult or aged; and in the skulls of the younger specimens it is broader, shorter, and the front edge is broader and more truncated or straight, with only a slight rounding at the sides. The position of the grinders as regards the front part of the zygomatic arch is a good character for the distinction of the species, especially if a series of skulls from animals of different ages, and from the same locality, of each species are compared together; and it is the same with the rooting of the grinders themselves. But when adult skulls of different species are compared together, the forms of the skulls are so altered, the grinders generally so worn and altered by age, and their position in different species so similar, that the distinction of the species then becomes more difficult. The flap of thick bald skin produced beyond the hinder toes varies in length as compared with the toes, in the length of it before it divides into lobes, and the length of the lobes themselves in different species, and thus affords characters for their separation; but it is difficult to determine the proper length of it and its parts from a preserved specimen in the Museum. It is apt to be unnaturally stretched in length and width by the preparer, and it shrinks as it dries long after it is placed in the Museum.