In Chiroptera the ossicles and especially the malleus much resemble those of shrews. The stapes is always normal in character, never becoming at all columelliform.
Primates. In Man and the Anthropoid Apes the malleus has a rounded head, a short neck, and the manubrium, a processus longus and a processus brevis. The incus consists of an anvil-shaped portion from which arises a long tapering process. The stapes has diverging crura and consequently a wide canal. The crura in other monkeys do not diverge so much as in man and anthropoid apes. The New World monkeys have no neck to the malleus.
The Sternum[168].
In Monotremes and most Marsupials the sternum does not present any characters of special importance. The presternum is strongly keeled in Notoryctes.
The sternum in Edentates is very variable: in the Sloths it is very long, the mesosternum of Choloepus having twelve segments. In the ant-eaters and armadillos the presternum is broad and sometimes as in Priodon strongly keeled. In Manis macrura the xiphisternum is drawn out into a pair of cartilaginous processes about nine inches long.
In the Sirenia the sternum is simple and elongated, and of fairly equal width throughout, in the adult it shows no sign of segmentation. Its origin from the union of two lateral portions can be well seen in Manatus.
Two distinct types of sternum are met with in the Cetacea. In the Odontoceti the sternum consists of a broad presternum followed by three or four mesosternal segments, but with no xiphisternum. Indications of the original median fissure can be traced, and are very evident in Hyperoödon. In the Mystacoceti, on the other hand, the sternum consists simply of a broad flattened presternum which is sometimes more or less heart-shaped, sometimes cross-shaped. Only a single pair of ribs are united to it.
The sternum in Ungulata is generally long and narrow and formed of six or generally seven segments. The presternum is as a rule small and compressed, often much keeled, especially in the horse and tapir. The segments of the mesosternum gradually widen as followed back and the xiphisternum is often terminated by a cartilaginous plate.
In the Rodentia the sternum is long and narrow and generally has a large presternum, and a xiphisternum terminated by a broad cartilaginous plate.
In the Carnivora, too, the sternum (fig. 76) is long and narrow and formed of eight or nine pieces, all of nearly the same size. The xiphisternum generally ends in an expanded plate of cartilage.