Among the Mystacocetes, in the especially distinguishing characters of the division, the Rorquals are less specialised than the Right Whales, which in the greater size of the head, the length and compression of the rostrum, the development of the baleen, and shortness of the cervical region, are exaggerated forms of the type, and yet they retain more fully some primitive characters, as the better development of the hind limb, the pentadactylous manus, and the absence of a dorsal fin. Both types are found distinct in a fossil state at least as far back as the early Pliocene age, but generally represented by smaller species than those now existing. Some of the Pliocene Rorquals (_Cetotherium_) were, in the elongated flattened form of the nasal bones, the greater distance between the occipital and frontal bone at the top of the head, and the greater length of the cervical vertebræ, more generalised than those now existing. In the shape of the mandible also, Van Beneden, to whose researches we are much indebted for a knowledge of these forms, discerns some approximation to the Odontocetes. Among the last-named group there are several distinct types, of which that represented by _Platanista_, although in some respects singularly modified, has been considered to present on the whole approximations towards the more normal and general type of mammalian structure. It is therefore interesting to find an apparently allied form well represented among the earliest fossil remains of Cetaceans in Europe. Almost all the other members of the suborder range themselves under the two principal heads of Ziphioids (or Physeteroids) and Delphinoids. The former is an ancient and once abounding type, of which the Sperm Whale (_Physeter_) is a highly specialised form. Among the latter, _Globicephalus_ is a modified form as regards the structure of its anterior extremity, and _Monodon_ as regards its dentition, while _Delphinus_, with the various allied genera, may be regarded as the dominating type of Cetaceans at the present day, abundant in slightly differentiated species and also in individuals. They are in this respect to the rest of the order much as the hollow-horned Ruminants are to the other Ungulates.