SPALACID2E.
Genus E h i z o m y s , Gray.
The genus Mhizomys forms a small natural group, allied, on the one hand, to
Arvicola, and, on the other, to Siphneus.
The dentition of these genera belongs to three types: in Mhizomys, the molar
teeth are always rooted, even before they have appeared externally, and this character
is retained through life, so that the crown, as the tooth is not a persistent grower,
is the subject of considerable modification, depending on the extent to which the
folds of enamel are worn down. In early stages, the anterior molar of M. chinensis,
Gray, presents as many as four distinct folds, one internal and three external; but as
age advances, and use wears them away, some portions of the folds form little islets
of enamel, depending on differences in their vertical distribution; but in the well-
worn teeth of old animals all traces of these folds and islets disappear, and the tooth
becomes cylindrical, with a smooth, slightly concave crown, surrounded by a raised
border of hard enamel. The dental conformation is much the same in all the species.
In Arvicola the molars are occasionally rooted, but in some of the species they are
unfurnished with roots in youth, to gain them, however, with advancing years, so
that this group serves to connect Hhizomys with its permanently rooted teeth, to the
allied genus Siphneus, in which the teeth are continuous growers.
However, there is a general type of dental structure which closely affines
Mhizomys to Arvicola and Siphneus, the molar teeth of all these groups being
resolvable to one type, but varying in the number and extent of the enamel folds;
but, in the two last, the folds are more open and simple than in Mhizomys. Yet if
the teeth of M. chinensis are compared with those of Siphneus a/rmandi/i, it will
be found that, although the former appear wonderfully complex, they can be
reduced to the same structural type characteristic of the latter. In M. chinensis, in
which the crowns of the molars are slightly worn, the plan of structure can be
clearly traced to the presence of generally one fold on the inside of each molar,
and three external folds on the first molar, two or three on the second, and one
external fold on the third. According as the tooth is worn, these elementary folds,
which are, so to speak, closely pressed together, form wavy outlines, at first more
complicated, but ultimately, as the enamel folds rub down by use, becoming a simple
circle of enamel. In Siphneus armandii, there is no complication of the elementary
folds, all the molar teeth conforming to one plan, viz., one internal and two external
folds; that is to say, that the first molar has one external fold less than in M. chinensis,
while the others agree in structure with the second and third molars of that
species. A similar uniformity of structure pervades the inferior molars in both
genera. The only difference presented by Arvicola is, that the folds are more open
and deep, and more numerous on the inner than on the outer side of the tooth,
There are also other characters, such as the form and internal structure of the
stomach and csecum, by which Mhizomys nearly approaches to Siphneus and Arvicola,
besides many strong resemblances to each other manifested by the skeletons of
the first two, and between their skulls and that of Arvicola. These considerations
which will be amplified in the following sketch of the structural peculiarities of
Mhizomys would seem to determine that this genus has its nearest allies in Siphneus
and Arvicola, a fact which A. M.-Edwards1 has pointed out and on the strength of
which he has instituted a small natural division represented by Arvicola, of which
he regards Siphneus as a derivative type, modified by the external circumstances in
which it is destined to live.
The leading feature of the vertebral column of Mhizomys, as in Siphneus, is,
the more than usual breadth and strength of the cervical vertebrae, developed doubtless
in relation to the heavy head and burrowing habits of the animal, but differing
from that genus in that there is no amalgamation of any of the cervical vertebrae.
The atlas has its spinous process reduced to two nodules, but in the axis this process
is massive, as in Siphneus, having considerable antero-posterior expansion. Erom the
third to the seventh cervical vertebra, Mhizomys, unlike Spalax, Oeorychus, Arvicola,
and Bathyergus, has the spinous processes well developed and gradually increasing
in size from before backwards. The transverse process of the atlas, contrary to that
of Siphneus, is flattened and of considerable extent, as in Bathyergus ; its lower or
anterior aspect being rather deeply concave, its lower division being prolonged downwards
as a well-marked ridge on to the inferior arch of the vertebra which bears a
nodular hypapophysis, as in Siphneus, much more reduced than that which occurs in
rats. Erom the axis to the sixth cervical, the two divisions of the transverse process
increase in size from before backwards, the lower portion being placed somewhat
anterior to the one above it, so that the artery is more protected than if the processes
lay directly over each other. The extremity of the sixth process, as in Siphneus, is
widely bifurcate, while only the upper division of the seventh process is developed.
The spinous process of the first dorsal little exceeds in vertical extent that of the
seventh cervical, but this process rapidly increases in length to the fourth dorsal,
where it attains a length of half an inch ; and as far as the twelfth dorsal, it is much
directed backwards, diminishing in height, but gaining in antero-posterior extension
to the penultimate lumbar.
Metapophyses begin to show themselves on the twelfth dorsal, and go on developing
in size as they are traced backwards to the last lumbar, being also well developed on
the first sacral and traceable throughout the sacral elements. Anapophyses begin on
the thirteenth dorsal, and attain their maximum size on the second lumbar, disappearing
on the last lumbar. The transverse process of the first lumbar vertebra exists
only as a rudiment, but this process increases in size to the last lumbar, but it does
not attain the same proportional development that it assumes in some Mwridoe. The
bodies of all these vertebræ are rounded on their under surfaces, and do not show
any of the ridge-like hypapophyses which characterise the murine skeleton; no
1 Rech. des Mammif. p. 78.