Suborder (4). Amblypoda[130].
This suborder includes a number of primitive extinct Ungulates, many of which are of great size. Their most distinguishing characteristics are afforded by the extremities. In the carpus the bones interlock a little more than is the case in most Subungulata, and the corner of the os magnum reaches the scaphoid, while the lunar articulates partially with both magnum and unciform, instead of only with the magnum. In the tarsus the cuboid articulates with both the calcaneum and the astragalus, which is remarkably flat. The manus and pes are short, nearly or quite plantigrade, and have the full number of digits. The cranial cavity is singularly small. Canine teeth are present in both jaws, and the grinding teeth have short crowns, marked by V-shaped ridges. The pelvis is large, the ilia are placed vertically, and the ischia do not take part in the ventral symphysis.
The best known animals belonging to this suborder are the Uintatheriidae (Dinocerata)[131], found in the Upper Eocene of Wyoming. They are as large as elephants, and are characterised by the long narrow skull drawn out into three pairs of rounded protuberances, by the strong occipital crest, and by the very large upper canines.
Suborder (5). Proboscidea.
This suborder includes the largest of land mammals, the Elephants, and certain of their extinct allies. The limbs are strong, and are vertically placed; the proximal segment is the longest, and the manus and pes are pentedactylate and subplantigrade. The digits are all enclosed in a common integument, and each is provided with a broad hoof. The vertebral centra are much flattened and compressed, especially in the cervical region. The number of thoracic vertebrae is very great, reaching twenty. The skull (figs. 96 and 97) is extremely large, this being due to the great development of air cells, which takes place in nearly all the bones of the adult skull. In the young skull there are hardly any air cells, and the growth of the cranial cavity does not by any means keep pace with the growth of the skull in general. The supra-occipital is very large, and forms a considerable part of the roof of the skull. The nasals and jugals are short, and the premaxillae very large. The rami of the mandible meet in a long symphysis, and the ascending portion is very high. Canine teeth are absent, and the incisors have the form of ever-growing tusks composed mainly of dentine; in living forms they are present in the upper jaw only. The grinding teeth are large, and in living forms have a very complex structure and mode of succession. In some of the extinct forms, such as Mastodon and especially Dinotherium, the teeth are much more simple. In every case the teeth have the same general structure, consisting of a series of ridges of dentine, coated with enamel. In the more specialised forms the valleys between the ridges are filled up with cement. The acromion of the scapula has a recurved process, similar to that often found in rodents. Clavicles are absent. The radius and ulna are not ankylosed, but are incapable of any rotatory movement. All the bones of the extremities are very short and thick; the scaphoid articulates regularly with the trapezoid and the lunar with the magnum. The ilia are vertically placed, and are very much expanded; the ischia and pubes are small, and form a short symphysis. The femur has no third trochanter, and the tibia and fibula are distinct. The fibula articulates with the calcaneum, and the astragalus is very flat.
Here brief reference may be made to the Tillodontia[132], a group of extinct mammals found in the Eocene beds of both Europe and North America. They seem to connect together the Ungulata, Rodentia, and Carnivora.
The skull resembles that of bears, but the grinding teeth are of Ungulate type, while the second incisors resemble those of rodents, and have persistent pulps. The femur has a third trochanter, and the feet resemble those of bears in being plantigrade and having pointed ungual phalanges, differing, however, in having the scaphoid and lunar distinct.
Order 5. Rodentia.
The Rodents form a very large and well-defined group of mammals easily distinguishable by their peculiar dentition. Canines are absent, and the incisors are very large and curved, growing from persistent pulps. They are rectangular in section and are much more thickly coated with enamel on their anterior face than elsewhere; consequently, as they wear down they acquire and retain a chisel-shaped (scalpriform) edge. There is never more than one pair of incisors in the mandible, and except in the Hares and Rabbits, there is similarly only a single pair in the upper jaw. These animals are, too, the only rodents which have well developed deciduous incisors. There is always a long diastema separating the incisors from the grinding teeth. The grinding teeth, which are arranged in a continuous series, vary in number from two to six in the upper jaw, and from two to five in the lower jaw. The number of premolars is always below the normal, often they are altogether wanting, but generally they are 1/1. Sometimes the grinding teeth form roots, sometimes they grow persistently.