The Shrew-mice, or, shortly, shrews (Soricidae), are closely related to the Talpidae, with which they are connected by means of some of the subfamily Myogalinae. They are, however, distinguished Shrews. by the ring-like tympanic, the incompleteness of the zygomatic arch, the tubercular-sectorial type of upper molar, the two-cusped first upper incisor, and the forward direction of the corresponding lower tooth. As a rule they are terrestrial, but a few are aquatic.
| Fig. 5.—Skull and Dentition of a Shrew-mouse (Sorex-verae-pacis); i, first incisors; c in the upper jaw is the canine; and p-m the three premolars, behind which are the three molars; in the lower jaw c is the second incisor, and p the single premolar. |
The dentition (fig. 5) is characteristic, and affords one of the chief means of classifying this exceedingly difficult group of mammals. There are no lower canines, and always six functional teeth on each side of the lower jaw, but in some rare instances an additional rudimentary tooth is squeezed in between two of the others. The first pair of teeth in each jaw differ from the rest; in the upper jaw they are hooked and have a more or less pronounced basal cusp; in the lower jaw they are long and project horizontally forwards, sometimes with an upward curve at the tip. Behind the first upper incisor comes a variable number of small teeth, of which, when all are developed, the first two are incisors, the third the canine, and the next two premolars; behind these, again, are four larger teeth, of which the front one is the last premolar, while the other three are molars. Thus we have in the typical genus Sorex (fig. 5) the dental formula i. 3⁄2, c. 1⁄0, p. 3⁄1, m. 3⁄3, total 32, or twenty upper and twelve lower teeth. The lower formula, as already stated, is constant, but the number of the upper series varies from the above maximum of twenty to a minimum of fourteen in Diplomesodon and Anurosorex, in which the formula is i. 2, c. 1, p. 1, m. 3. From the relation of the fourth upper tooth to the premaxillo-maxillary suture it has been supposed that shrews, like many polyprotodont marsupials, have four pairs of upper incisors; but this is improbable, and the formula is accordingly here taken to follow the ordinary placental type.
Shrews may be divided into two sections, according as to whether the teeth are tipped with brownish or reddish or are wholly white, the former group constituting the Soricinae and the latter the Crocidurinae.
In the red-tipped group is the typical genus Sorex, which ranges over Europe and Asia north of the Himalaya Mountains to North America. There are twenty upper teeth with the formula given above, the ears are well developed, the tail is long and evenly haired, and the aperture of the generative organs in at least one of the sexes is distinct from the vent. The common shrew-mouse (Sorex araneus) has a distribution co-extensive with that of the genus in the Old World, and the North American S. richardsoni can scarcely be regarded as more than a local race. A few species, such as Sorex hydrodomus of Alaska and S. palustris of the United States, have fringes of long hairs on the feet, and are aquatic in habit. The latter has been made the type of the genus Neosorex, but such a distinction, according to Dr J. E. Dobson, is unnecessary. The same authority likewise rejects the separation of the North American S. bendirei as Atophyrax, remarking that this species is an inhabitant of marshy land, and appears to present many characters intermediate between S. palustris and the terrestrial species of the genus, differing from the former in the absence of well-defined fringes to the digits, but agreeing with it closely in dentition, in the large size of the infra-orbital foramen, and in the remarkable shortness of the angular process of the lower jaw. In India and Burma the place of Sorex is taken by Soriculus, in which the upper teeth are generally 18, although rarely 20, and the generative organs have an opening in common with the vent after the fashion of the monotreme mammals. The latter feature occurs in the North American Blarina, which is characterized by the truncation of the upper part of the ear and the short tail, the number of upper teeth being 20 or 18. Another American genus, Notiosorex, in which the ear is well developed and the tail medium, has only 16 upper teeth. From all the rest of the red-toothed group the water-shrew, Neomys (or Crossopus) fodiens, of Europe and northern Asia, differs by the fringe of long hairs on the lower surface of the tail; the number of upper teeth being 18.
In the white-toothed, or crocidurine, group, the small African genus Myosorex, which has either 18 or 20 upper teeth, includes long-tailed and large-eared species in which the aperture of the generative organs and the vent, although close together, are yet distinct. In the musk-shrews (Crocidura), on the other hand, which are common to Europe, Asia and Africa, the reproductive organs and the alimentary canal discharge into a common cloaca, the long tail is sparsely covered with long and short hairs, there are anal glands secreting a strong musky fluid, and the number of upper teeth is 16 or 18. Diplomesodon pulchellus of the Kirghiz steppes, has, on the other hand, only 14 upper teeth, and is further characterized by the moderately long tail and the hairy soles of the hind-feet. Another genus is represented by the Tibetan Anurosorex squamipes, which has the same dental formula, but a mole-like form, rudimentary tail and scaly hind-soles. Lastly, we have two Asiatic mountain aquatic species, Chimarrogale himalayaca of the Himalayas and Nectogale elegans of Tibet, which have fringed tails like the European water-shrew, and 16 upper teeth, the former characterized by the small but perfect external ears, and the latter (fig. 6) by the absence of the ears and presence of adhesive disks on the feet.
It will be seen that the red- and the white-toothed series have parallel representative forms, which may indicate that the division of the family into the two groups is one based rather on convenience than on essential differences. See [Shrew].
From the shrews, the hedgehogs and gymnuras, or rat-shrews, collectively forming the family Erinaceidae, differ structurally by the broader ring made by the tympanic, the complete zygomatic arch, the five-cusped broad upper molars, and the presence of a short pubic symphysis. At the present day they are an exclusively Old World group.
| Fig. 6.—The Tibetan Water-shrew (Nectogale elegans). |
The typical group, or Erinaceinae, is represented only by the hedgehogs, with the one genus Erinaceus, easily recognized by their spiny coats, and further characterized by the rudimentary tail, the presence of vacuities in the palate, and the broad Hedgehogs. pelvis. Hedgehogs (Erinaceus) have the dental formula i. 2⁄2, c. 1⁄1, p. 3⁄2, m. 3⁄3, and are represented by over a score of species, distributed throughout Europe, Africa and the greater part of Asia, but unknown in Madagascar, Ceylon, Burma, Siam, the Malay countries, and, of course, Australia. All the species resemble one another in the armour of spines covering the upper surface and sides of the body; and all possess the power of rolling themselves up into the form of a ball protected on all sides by these spines, the skin of the back being brought downwards and inwards over the head and tail so as to include the limbs by the action of special muscles.