Section II. Subungulata.
In this group is placed a heterogeneous collection of animals, the great majority of which are extinct. There is really no characteristic which is common to them all, and which serves to distinguish them as a group from the Ungulata vera. But the most distinctive character common to the greatest number of them is to be found in the carpus, whose bones in most cases retain their primitive relation to one another, the os magnum articulating with the lunar and sometimes just meeting the cuneiform, but in living forms at any rate not articulating with the scaphoid. The feet frequently have five functional digits, and may be plantigrade. The proximal surface of the astragalus is generally flattened instead of being pulley-like as in Ungulata vera.
Suborder (1). Toxodontia.
This suborder includes some very aberrant extinct South American ungulates, which have characters recalling the Proboscidea, both groups of Ungulata vera, and the Rodentia. The limbs are subplantigrade or digitigrade, and the digits are three, rarely five, in number, the third being most developed. The carpus resembles that of the Ungulata vera, in that the bones interlock and the magnum articulates with the scaphoid. In the tarsus, however, the bones do not interlock. The astragalus has a pulley-like proximal surface (except in Astrapotherium, in which it is flat), and articulates only with the navicular, not meeting the cuboid. The calcaneum has a large facet for articulation with the fibula, as in Artiodactyla. There is no alisphenoid canal, and the orbit is confluent with the temporal fossa. Some of the forms (e.g. Nesodon) referred to this group have the typical mammalian series of forty-four teeth, but in others the canines are undeveloped. In Toxodon all the cheek teeth have persistent pulps, while in Nesodon and Astrapotherium they are rooted. A clavicle is sometimes present (Typotherium), and the femur sometimes has a third trochanter (Typotherium and Astrapotherium), sometimes is without one (Toxodon).
The remains of these curious Ungulates have been found in beds of late Tertiary age in South America.
Suborder (2). Condylarthra[128].
This group includes some comparatively small extinct ungulates, which are best known from the Lower Eocene of Wyoming, though their remains have also been found in deposits of similar age in France and Switzerland. Their characters are little specialised, and they show relationship on the one hand to the Ungulata vera and on the other to the Hyracoidea. They also have characters allying them to the Carnivora. They generally have the typical mammalian series of forty-four teeth, the molars being brachydont and generally bunodont. The premolars are more simple than the molars. The limbs are plantigrade, and have five digits with rather pointed ungual phalanges. The os magnum, as in living Subungulates, articulates with the lunar, not reaching the scaphoid. The astragalus has an elongated neck, a pulley-like proximal and a convex distal articular surface, and does not articulate with the cuboid. The humerus has an ent-epicondylar foramen, and the femur has a third trochanter. The best known genus is Phenacodus; it is perhaps the most primitive ungulate whose skeleton is thoroughly well known, and is of special interest from the fact that it is regarded as the lowest stage in the evolutionary series of the horse. Its remains are found in the Lower Eocene of Wyoming.
Suborder (3). Hyracoidea[129].
This group of animals is very isolated, having no very close allies, either living or extinct. The digits are provided with flat nails, except the second digit of the pes, which is clawed. Canine teeth are absent, and the dental formula is usually given as i ½, c 0/0, pm 4/4, m 3/3. The upper incisors are long and curved, and have persistent pulps as in Rodents; their terminations are, however, pointed, not chisel-shaped, as in Rodents. The lower incisors have pectinated edges. The grinding teeth have a pattern much like that in Rhinoceros. In the skull (fig. 83) the postorbital processes of the frontal and jugal almost or quite meet. The jugal forms part of the glenoid cavity for articulation with the mandible, and also extends forwards so as to meet the lachrymal. There is an alisphenoid canal. There are as many as twenty-one or twenty-two thoracic vertebrae, and the number of thoraco-lumbar vertebrae reaches twenty-eight or thirty. There are no clavicles, and the scapula has no acromion; the coracoid process is, however, well developed. The ulna is complete. In the manus the second, third and fourth digits are approximately equal in size, the fifth is smaller, and the first is vestigial. The femur has a slight ridge representing the third trochanter. The fibula is complete, but is generally fused with the tibia proximally. There is a complicated articulation between the tibia and astragalus, which has a pulley-like proximal surface. In the pes the three middle digits are well developed, but there is no trace of a hallux, and the fifth digit is represented only by a vestigial metatarsal.
The only representatives of the suborder are some small animals belonging to the genus Procavia (Hyrax), which is found in Africa and Syria; some of the species are by many authors placed in a distinct genus Dendrohyrax.