In Sirenia the general structure of the manus is quite of the ordinary mammalian type. In Manatus most of the bones of the carpus are distinct, but in Halicore many, especially those of the distal row, have coalesced. The digits are always five in number and have the normal number of flattened phalanges.
In the Cetacea, on the other hand, the manus is much modified by the fact that the number of phalanges may be greatly increased above the normal number of three, thirteen or fourteen sometimes occurring in each digit. These are believed to be duplicated epiphyses. In the Mystacoceti the manus remains largely cartilaginous, in the Odontoceti it is better ossified, and the phalanges commonly have epiphyses at both ends. In Physeter the carpal bones also have epiphyses. The carpus generally consists of six bones arranged in two rows of three each. Five digits are generally present, but sometimes as in Balaenoptera musculus, there are four, the third being suppressed. Their relative development varies much. The Sperm Whale which till recently was placed in the entrance hall of the Natural History Museum at South Kensington has one phalanx to the first digit, four to the second, five to the third, four to the fourth, and three to the fifth. Generally the manus is short and broad, but sometimes, as in Globicephalus, it is much elongated owing to the great development of the second and third digits.
Ungulata[173]. The manus of the members of this great order is of very great classificatory and morphological importance. All the members agree in having the scaphoid and lunar distinct, and in almost every case the ends of the digits are either encased in hoofs or provided with broad flat nails. It is by means of characters derived from the manus and pes that the group is subdivided into the Ungulata vera and the Subungulata.
In the Ungulata vera the manus is never plantigrade, and there are not more than four digits, the pollex being almost always completely suppressed: in Cotylops among extinct Artiodactyla however a vestigial pollex is found. The centrale is absent, and the magnum articulates freely with the scaphoid, and is separated from the cuneiform by the unciform and lunar. All the bones of the carpus interlock strongly, and the axis of the third digit passes through the magnum and between the scaphoid and lunar.
There is a very strong distinction between the manus of the suborders Artiodactyla and Perissodactyla. In the Artiodactyla the axis of the manus passes between the third and fourth digits, which are almost equally developed and, except in the Hippopotami and some extinct forms such as Anoplotherium, have their ungual phalanges flattened on their contiguous surfaces.
In all Artiodactyla the third and fourth digits are large, but a gradual reduction in the second and fifth can be well traced. Thus in the Suina the second and fifth digits, though smaller than the third and fourth, are well developed and all four metacarpals are distinct. In the Tragulina too all four metacarpals are developed, and in Dorcatherium the third and fourth commonly remain distinct as in the Suina. In the other Artiodactyla however the third and fourth metacarpals are almost always united, though indications of their separate origin remain. In some Ruminantia, such as many Deer, the second and fifth digits are reduced to minute splint bones attached to the proximal end of the fused third and fourth metacarpals, and to small hoof-bearing phalanges, sometimes attached to splint-like distal vestiges of the metacarpals, sometimes altogether unconnected with any other skeletal structures. In some other Ruminants, such as the Sheep and Oxen, the only remnants of the second and fifth digits are nodules of bone supporting the hoofs, and in others, such as the Giraffe, Anoplotherium commune, some Antelopes and the Tylopoda, all traces of these digits have disappeared. The Camels differ from all living Ungulata vera in not having the distal phalanges completely encased in hoofs, and from all except the Hippopotami in placing a considerable amount of the manus on the ground in walking.
Fig. 106. Manus of Perissodactyles.
A. Left Manus of Tapirus. (After von Zittel.)