mountain. Externally it is something like the Pikas, since it has no tail visible. The ears, too, are short, and the hind-legs comparatively short. The skull is very like that of the Rabbit; but in other osteological details it is aberrant. Thus the clavicle is quite complete, and only six ribs articulate with the sternum, instead of the seven that we find in the Rabbit.
Fam. 2. Lagomyidae.—The animals of this family are smaller than the Hares and Rabbits; they have short Vole-like ears and no external tail. The limbs also appear to be shorter. As there is but a single genus, the characters of the family may be described in connexion with those of the genus, which is known as Lagomys (apparently more correctly Ochotona). Of this genus there are about sixteen species, which are mainly Asiatic; one species extends its range into Eastern Europe, and three are North American.
The skull has not the supra-orbital grooves of the Rabbits, and has a well-marked backward process of the zygomatic arch. There are eighteen dorsal vertebrae. The molars and premolars are five.
The vernacular names of "Pika" and "Piping Hares" have been applied to the members of this genus, the latter on account of their peculiar call. They live among rocks in companies and they burrow. They are usually found at considerable altitudes: thus L. roylei, the "Himalayan Mouse Hare," is found at elevations as high as 16,000 feet; while L. ladacensis gets even higher, 19,000 feet having been recorded. With the habits of a Marmot, so far as concerns living in burrows and at great altitudes, the animals of this genus, with their squat form and short ears, are not unlike those animals. In the past this genus occurred more generally over Europe. Species from Miocene beds have been met with in England, France, Germany, and Italy.
Fossil Rodents.—Quite a large number of existing genera of Rodents are known from even the earlier strata of the Tertiary period. The Squirrels (and even the genus Sciurus itself) occur in the Upper Eocene. So, too, do the genera Myoxus, and (in South America) Lagostomus. Spermophilus, Acomys, Hystrix, Lagomys, Lepus, Hesperomys are known from Miocene rocks. Rhizomys, Castor, Cricetus, Mus, Microtus, and some others appear to have originated so far as we know in the Pliocene, while a still larger series of existing genera are Pleistocene. It is interesting
to note that some of the extinct genera were much larger than recent forms. At present, Hydrochoerus is the biggest Rodent; but the genus Megamys from the Pampas formation of Argentina was "nearly as large as an ox." The wider range of genera in the past is illustrated by Hystrix, which, now an Old-World form, is represented by remains in the Miocene and Pliocene of America.
It is a significant fact that of living genera Sciurus is the oldest; for it has been pointed out that in a number of features the Squirrels are among the most primitive of Rodents. The zygomatic arch is slender, and has thus not acquired the specialisation that is to be found in that part of the skull in other Rodents; moreover, the "jugal bone is not supported by any process from the maxilla exactly as in the primitive Ungulata." The feet, too, are unspecialised, though that is the case with many other genera. It may also be pointed out that the teeth bear not a little likeness to those of Ornithorhynchus in their multituberculate character.
Some few fossil forms have already been dealt with in the preceding pages.
The two genera Castoroides and Amblyrhiza, from the Pleistocene of North America and the West Indies, are usually regarded as forming a family. The skull of the former genus indicates an animal of the size of a Bear. It is compared to that of Castor, but it has a wide infra-orbital foramen. The teeth are four in each jaw, and are formed of three to five lamellae; the incisors of this animal are powerful but short. Amblyrhiza, on the other hand, has long incisors which are longitudinally grooved anteriorly. It has a free fibula. This latter as well as other characters have led Tullberg to remove it from association with Castoroides.