In MONOTREMES the ossicles, though distinctly mammalian in character, show a very low type of development. The incus is articulated, or often fused, with an outgrowth from the head of the malleus. The stapes is very much like a reptilian columella, having a single crus with no perforation. In MARSUPIALS the ossicles are of a low type, but not so low as the rest of the skeleton might have led one to expect, and all or almost all the points showing a low grade of development may be paralleled among the Monodelphia. The lowest Marsupials as regards the ossicles are the Peramelidae, whose ossicles are of a frail papery consistence. The Didelphyidae on the other hand have the most highly developed ossicles, the malleus much resembling that of many Insectivores, and the stapes having two definite crura separated by a canal. In EDENTATES the character of the ossicles varies much. In Sloths the stapes approaches that of Sauropsids in its narrowness and the slight trace of a canal; this character is however still more marked in _Manis_, whose stapes is as Sauropsidan as that of Monotremes, and consists of a nearly circular basal plate bearing a column which does not show any sign of division into crura. The stapes of other Edentates, such as ant-eaters, aard varks, and most armadillos, is of a high type and has well-developed crura. _Priodon_ has a lower type of stapes than _Dasypus_ and _Tatusia_. The ossicles of the SIRENIA differ widely from those of all other mammals in their great density and clumsy form. In CETACEA the ossicles are solid, though not so solid as in Sirenia, and their details vary much. The malleus is always firmly fused to the tympanic by means of the processus longus, and the manubrium is very little if at all developed. The incus has the stapedial end greatly developed, and the stapes has very thick crura with hardly any canal. The ossicles of the Mystacoceti are apparently less specialised than are those of the Odontoceti. The auditory ossicles of the UNGULATA do not present any characters common to all the members of the group. Among Ruminants they are chiefly remarkable for the development of a broad lamellar expansion between the head and the processus longus of the malleus. In some cases the malleus of the foetus differs strikingly from that of the adult. Among Perissodactyla the Rhinoceros and Tapir have the malleus of a low type, recalling those of Marsupials; while in the Horse the head is well developed, and the malleus is of a higher type. The ossicles of _Procavia_, which recall those of the Equidae, are chiefly remarkable for the small size of the body of the incus. In Elephants the ossicles are large and massive. In the RODENTIA (fig. 100, C) the malleus is generally characterised by a very broad manubrium. In many genera such as _Bathyergus_, and most of the Hystricomorpha such as _Hystrix_, _Chinchilla_ and _Dasyprocta_, the malleus and incus are ankylosed together.