Fossorial or Digging Types have been developed usually from scampering short-footed types with short powerful arms. In the mole, an extreme and unique fossorial type, the fore-limbs have been moved forward under the neck to enable the enormous, wide hands to reach in front of the nose. The humeri have acquired a secondary contact with the clavicles, which have become solid blocks that form the pivots of the forearm and rest on a keel shaped forward extension of the sternum. The scapulae are elongate, to give long shoulder muscles and a long reach of the humeri ; the enormous triceps is inserted into a great hooked olecranon process of the ulna, the lower surface of which sup ports the powerful flexors of the carpus. The hind limb is small and not much modified.
The Cape golden moles (Chrysochloridae) differ widely from the true moles. Their forefeet are very narrow and adapted for digging in hard soil, with one or two sickle-like claws. The arma dillos and many rodents dig with their large claws and powerful f ore-limbs.
Natatorial or Swimming Types have been developed in many orders, although the extreme form of aquatic adaptation is lim ited to the Cetacea and Sirenia (q.v.). The end-results of pro longed aquatic adaptation naturally depend partly on what type of terrestrial forms the given aquatic types took rise from. Thus among the marsupials the aquatic opossum or yapok is simply an opossum with webbed feet. Among the Mustelidae the ordinary otter, the African Aonyx and the sea-otter (Latax) represent pro gressive adaptation to aquatic life, in which the feet become webbed and enlarged. The sea-lions carry this line of adaptation much further, greatly enlarging the hands and feet into flippers, but retaining the power to support the body on all four limbs. Finally, the earless seals (Phocidae) have lost the power of bring ing the hind limbs forward under the body and all four feet are specialized paddles. The body, enclosed in a thick layer of fat, has become streamlined and in the cetaceans becomes fish-like. These specializations were already under way in the oldest known whales, the Eocone Archaeoceti. Nevertheless, evidence from comparative anatomy and embryology proves that the Cetacea have been derived from terrestrial, quadrupedal placentals, per haps allied with the stem of the insectivores and carnivores. The manatees and dugongs (Sirenia) approach the Cetacea in aquatic specialization, but the resemblance is due to convergence as the anatomical differences indicate that the Sirenia have been derived from herbivorous ancestors, possibly related to the ances tral elephants.
Scansorial or Climbing Types.—Many small mammals with well-developed claws and spreading hands and feet supplied with interdigital pads can climb tree-trunks. In the marsupials the opossums afford an example of a primitive stage of arboreal spe cialization. The phalangers illustrate a more advanced type of highly specialized hind feet ; the great toe is strongly divergent and flattened, the second and third toes slender, closely appressed and enclosed in a common skin, the fourth and fifth enlarged and forming the outer fork of a clamp, the great toe forming the inner.
The tree-shrews (Tupaidae), which are probably very primitive Primates, give the initial stages of arboreal adaptation in that order. The pen-tailed tree-shrew (Ptilocercus) of Borneo has small spreading hands and feet and a generalized skeleton. The lemurs and their Eocene forerunners Adapidae and Notharctidae, have grasping hind-feet with a wide flat nail on the large great toe; the digits are elongate, slender with small nails, except the second digit, which bears a small claw. Lemurs are essentially arboreal quadrupeds, running along the tops of the branches, and the same is true of the typical monkeys. The New World mon keys, which have a prehensile tail, can also hang and swing from the branches. The monkeys, especially those of the Old World division, sit upright, or partly so, resting on the ischial callosities. The hands are used in manipulating the food.
Brachiating or Acrobatic Forms.—The gibbons do not run on all fours but hold the body erect, raising the long arms above the head. They make long leaps through the air, catching the branches. In their skeleton as well as in their internal anatomy they are closer to man than to the lower primates.
Of the larger anthropoids the orang-utan (q.v.) is extremely specialized for arboreal life, using the suspension grasp of the hands and feet. The arms are excessively long, with enfeebled thumbs, the legs very short. Progression on the ground is awk ward, the long arms being used as crutches.
The chimpanzee (q.v.) is a moderately heavy-bodied brachiat ing form, less specialized than the orang. In running on the ground a secondarily quadrupedal gait is employed, the weight of the heavy forepart of the body resting on the bent knuckles. Never theless tame chimpanzees, when carrying large objects in their arms, walk erect.