278 V L V L that they are worn down by use, just as is the case with the incisors, whose rootless condition is a characteristic of the whole Rodent order. Each tooth consists of a variable number of prismatic pillars formed externally of enamel and internally of the softer dentine, their section, as produced naturally by wear, being as is shown in the woodcut, a figure which may be compared with that given of the true murine dentition under MOUSE (vol. xvii. p. 5). The general prismatic appearance of the teeth, how ever, is only due to the bending in at regular distances of the external enamel walls of the tooth, to such an extent that the enamel touches that at the opposite side of the tooth and thus shuts off a greater or less number of dentinal spaces, triangular in section. Owing to the manner of growth of the teeth their general pattern of spaces and angles is but little affected by age and wear, remaining practically the same throughout the life of the animal. On this account the variations in the pattern have been very generally used as a means of characterizing the different genera, subgenera, and species of the group, although their value for this purpose has of late been questioned, and a new classification proposed, entirely ignoring the tooth- characters, and based more upon external peculiarities. Without entering into the characters of the different sub-groups of the voles, it may suffice to say that each of the three species found in Great Britain belongs to a dis tinct subgenus of Arvicola, and therefore the three together give a fair idea of the extent to which the subgenera differ from one another. These three species are the following : 1. The common Water- Vole (Arvicola ampliibius] is as large as the house-rat, with which it is so often confused, but possesses of course the bluff-headed appearance and short tail characteristic of the voles. Its fur is long, soft, and thick, of a uniform grizzled brown all over, except when, as is not uncommon, it is wholly black. Its tail is about half the length of its head and body, and its hind feet are unusually long and powerful, although not webbed, and have five rounded pads on their lower surfaces. Its molar teeth (see woodcut) present the following number of prismatic spaces: in the upper jaw the first, or anterior, has 5, the second 4, and the third 4, of which the last is very irregular in shape, and is sometimes itself divided into two, making 5 in all ; in the lower jaw the first has 7 spaces, of which the 3 anterior are generally not fully separated from one another, the second has 5, and the third 3. These numbers for the different teeth are taken as the characters of the subgenus Paludicola of Dr Blasius, by whom this method of subdividing the group was first introduced. The water-vole is one of the commonest English mammals, and is perhaps the most often actually seen of all, owing to its diurnal instead of nocturnal habits. It frequents rivers and streams, burrowing deeply into their banks, and in this way often causing considerable damage. Its food consists almost wholly of water- weeds, rushes, and other vegetable substances, but, like so many other rodents, it will also eat animal food on occasion, in the shape of insects, mice, or young birds. The female has during the warm season of the year three or four litters, each of from two to seven young. The range of the water-rat extends over the whole of Europe and North Asia, from England to China, but is not found in Ireland, where, curiously, no species of Arvicola is indigenous. 2. The next British species, representing the subgenus Agricola of Dr Blasius, is the common Field- Vole, or short-tailed field-mouse (Arvicola ayrcstis), about the size of a house-mouse, but with a short stumpy body, and a tail only about one-third the length of the head and body combined. Its hind feet have six pads on their inferior surfaces. Its colour is dull grizzled brown above, and greyish -white below. Its molar teeth have respectively 5, 5, and 6 prismatic spaces above, and 9, 5, and 3 below. The field-vole is one of the commonest of our smaller mammals, and frequents fields, woods, and gardens in enormous numbers, often doing very considerable damage in the latter owing to its fondness for garden produce of all kinds. It is spread over the whole of Great Britain from the Hebrides southwards. Abroad its range extends from Finland to North Italy and from France and Spain to Russia. 3. The Bank- Vole (Arvicola glareolus) is of much the size and general appearance of the common field-vole, but may be dis tinguished by its more or less rusty or rufous- coloured back, its larger ears, and its comparatively longer tail, which attains to about half the length of the head and body. Its molar teeth present characters so different from those of all other voles as to have caused it to be often looked upon as belonging to an entirely distinct genus, for which the name of Evotomys is used. Their chief distinction lies in the fact that, unlike those of all other voles, their pulp-cavities close up in adult life, and they form distinct roots, more resembling those of the ordinary rats and mice. The enamel spaces of these teeth number respec tively 5, 4, and 5 above, and 7, 3, and 3 below. The habits of this species are in every way similar to those of the field-vole. Its range in Great Britain extends northwards to Morayshire, beyond which it has not yet been observed. Abroad it is found all along the north temperate zone from France to China, and is replaced in North America by a closely allied animal known as Arvicola gappcri. It is probable, however, that both these forms, A. gapperi and A. glareolus, are only southern climatic offshoots of a still more northern species, the Arvicola rutilus of northern Europe, Siberia, and Arctic America. The foreign species of vole number about 40, of which about 10 are European, 20 Asiatic, and 10 North American, none being found either in Africa, India (except in the extreme north), Australia, or South America. The group is therefore one peculiarly characteristic of the temperate parts of the northern hemisphere. (o. T.) VOLGA, THE, the chief river of European Russia, rises See vol. in the Valdai plateau of Tver in north-western Russia, xxi. and after a winding course of 2325 miles (1040 in a Plate n - straight line) falls into the Caspian at Astrakhan. It is by far the largest river of Europe, those next in length, the Danube and the Ural, being only 1735 and 1478 miles respectively, while the Rhine (825 miles) is shorter even than two of the chief tributaries of the Volga, the Oka and the Kama. Its drainage area, which includes the whole of middle and eastern as well as part of south eastern Russia, amounts to 563,300 square miles, thus exceeding the aggregate superficies of Germany, France, and the United Kingdom. A hundred tributaries of the Volga are navigable for an aggregate length of 14,600 miles, a distance greater than the aggregate length of all the railways of England and Wales. The drainage area embraces twenty-one provinces of the Russian empire, or, in other words, nearly the whole of Great Russia proper, and has a population of nearly 40,000,000. The most populous regions of Great Russia are situated within the Volga basin, and cities like Moscow, JSTijni-Novgorod, Saratoff, Simbirsk, and Kazan, as well as many others, are indebted for their growth and present importance to their situation on the Volga or its tributaries. But the real " basin " of the Volga is not limited to its drainage area. By a system of canals which connects the upper Volga with the Neva, the commercial mouth of the Volga has been transferred, so to speak, from the Caspian to the Baltic, thus making St Petersburg, the capital and chief seaport of Russia, the chief port of the Volga basin as well. Other less important canals connect it with the Diina and the White Sea (Riga and Archangel) ; while a railway only 40 miles in length joins the Volga with the Don and the Sea of Azoff, and three great trunk lines bring its lower parts into connexion with the Baltic and western Europe. Traffic on the river and its tributaries is carried on by more than 760 steamers, 20,000 barges and boats, and 50,000 rafts; upwards of 4,000,000 tons of various goods, valued at 500,000,000 roubles (in weight one-half and in value three-fourths of the total merchandise on Russian rivers) are carried. If in addition some 50,000 rafts of timber not included in the preceding statistics are reck oned, the timber floated in the basin of the Volga exceeds by nearly a million tons the total weight of merchandise carried on the 13,000 miles of Russian railways. 1 The Volga rises in the extensive marshes covering the western parts of the Valdai plateau, where the Diina also has its origin. Small streamlets languidly circulate from marsh to marsh, so that 1 See "The Volga as a Means of Communication," by Prof. Boguslavskiy, St Petersburg, 1887 (Russian), being vol. ix. of the Sbornik of the Institute of Roads and Communications. The above length of the Volga is taken from this work, based on recent surveys, and is therefore more accurate than the length given by M. Strelbitzky
(1978 miles) aud Gen. Tillo (2108 miles).