Page:EB1911 - Volume 14.djvu/671

Rh form of what is known as the tritubercular sectorial type. There is no caecum.

The first representatives of this group are the moles, or Talpidae, in which the lower ends of the tibia and fibula are united (fig. 3, t, fb), there is a descent of the testes, the tympanum forms a bladder-like bulla, the zygomatic, or cheek-arch, although slender, is complete, there is no pelvic symphysis, the upper

molars are five-cusped, and the first upper incisor is simple, and the lower vertical. In habits the majority of the family are burrowing, but a few are aquatic; and all feed on animal substances. The distribution is limited to the temperate regions of Europe, Asia and North America.

Throughout the family the eyes are minute, and in some species are covered with skin; the ears are short and hidden in the fur; and the fore-limbs are generally more or less modified for digging.

The true moles of the genus Talpa are the typical representatives of the first subfamily, or Talpinae, in which the clavicle (fig. 3, cl.) and humerus (h) are very short and broad, while there is an additional sickle-like bone (fc) on the inner side of the fore-foot. In Talpa itself the first upper incisor is but little larger than the second, the fore-foot is very broad, and the dental formula is i., c. $1⁄1 or 0$, p., , or , m.. There are about a dozen species, all confined to the Old World. The variation in the dental formula of some of the best known of these is as follows:—

Except in T. europaea, the eyes are covered by a membrane. In T. micrura the short tail is concealed by the fur. T. europaea extends from England to Japan.

T. caeca and T. romana are found south of the Alps, the remaining species are all Asiatic, two only—T. micrura and T. leucura—occurring south of the Himalaya.

The genus may be split up into subgenera corresponding with the above table; these subdivisions being sometimes accorded full generic rank. For instance the Japanese T. wogura and the Siberian T. robusta are often referred to under the ill-sounding titles of Mogera wogura and M. robusta.

Referring more fully to the European species, it may be mentioned that the mole exhibits in its organization perfect adaptation to its mode of life. In the structure of the skeleton striking departures from the typical mammalian forms are noticeable. The first sternal bone is so much produced as to extend forward as far as a vertical line from the second cervical vertebra, carrying with it the very short almost quadrate clavicles, which are articulated with its anterior extremity and externally with the humeri, being also connected ligamentously with the scapula. The fore-limbs are thus brought opposite the sides of the neck, and from this position a threefold advantage is derived:—in the first place, as this is the narrowest part of the body, they add little to the width, which, if increased, would lessen the power of movement in a confined space; secondly this position allows of a longer fore-limb than would otherwise be possible, and so increases its lever power; and, thirdly, although the entire limb is relatively short, its anterior position enables the animal, when burrowing, to thrust the claws so far forward as to be in a line with the end of the muzzle, the importance of which is evident. Posteriorly, we find the hind-limbs removed out of the way by approximation of the hip-joints to the centre line of the body. This is effected by inward curvature of the innominate bones at the acetabulum to such an extent that they almost meet in the centre, while the pubic bones are widely separated behind. The shortness of the fore-limb is due to the humerus, which, like the clavicle, is so reduced in length as to present the appearance of a flattened X-shaped bone, with prominent ridges and deep depressions for the attachments of powerful muscles. Its upper extremity presents two rounded prominences; the smaller, the true head of the bone, articulates as usual with the scapula; the larger, which is the external tuberosity rounded off, forms a separate joint with the end of the clavicle. This double articulation gives the rigidity necessary to support the great lateral pressure sustained by the fore-limb in excavating. The bones of the fore-leg are normal, but those of the fore-foot are flattened and laterally expanded. The great width of the fore-foot is also partly due to the presence of a peculiar bone on the inner side of the palm and articulating with the wrist.

The muscles acting on these modified limbs are homologous with those of cursorial insectivora, differing only in their relative development. The tendon of the biceps traverses a long bony tunnel, formed by the expansion of the margin of the bicipital groove for the insertion of the pectoralis major muscle; the anterior division of the latter muscle is unconnected with the sternum, extending across as a band between the humeri, and co-ordinating the motions of the fore-limbs. The teres major and latissimus dorsi muscles are of immense size, inserted into the prominent ridge below the pectoral attachment, and are the principal agents in the excavating action of the limb. The cervical muscles connecting the slender scapulae, and through them the fore-limbs, with the centre line of the neck and with the occiput are large, and the ligamentum nuchae between them is ossified. The latter condition appears to be due to the prolongation forwards of the sternum, preventing flexion of the head downwards; and accordingly, the normal office of the ligament being lost, it ossifies,