In the Rodentia, the pelvis is largely de veloped for the support of the powerful hinder extremities in leaping, the most usual mode of progression of the generality of these animals.
The sacrunz is generally continued in a line vvith the lumbar curve in the long-tailed species. In Hares and Rabbits, however, the sacrum is considerably arched, longitudinally as well as transversely (fig. 98. a), and its angle with the spine marked 160°. There are generally four sacral vertebrx, but the first is much larger than the others, especially in its lateral masses (b), to articulate with the ilia. The Rabbit and Jerboa, however, present only two; and the common Rat and Beaver three; while the number in the Marmot is as many as six. The spines are not coalesced except in some Rats. The caudal vertebrm are more or less numerous, and are remarkable in the Beaver for the great length of the transverse processes and anterior spines, for muscular hold on this its useful appendage. In the Squirrel and Jerboa also, the tail is long and strong, and in the habitual sitting posture of these animals it forms, with the ischia, the third leg of a tripod, on which the body is sustained. In the short-tailed Rodents, the caudal ver tebrx are curved dorsally, in an opposite di rection to the coceygeal bones in Man and the Sinziz The ilia are long, prismatic, and slender in the shaft, having a central ridge passing up wards from the cotyloid cavity, with a groove on each side of it externally, and continued forward into elongated alm, little more ex panded than the shaft in most of the order. They form with the spine an angle of about 165° in the Hare, and 150° in the Porcupine. In the Copyrus, Rats, Mice, and Guinea-pig., they are nearly parallel with the spine ; but in the Jerboa they cross the spine at an angle much less than in others, being about 140°. In the Jerlxia the iliac wings are curved out wards superiorly and projected much on the dorsum of the spine, reaching beyond the elongated spines of the last lumbar vertebra. The ilia of the Beaver, and, in a less degree, in the Hare, are expanded, with prolonged and irregularly curved crests (e), a little everted at the spinous processes (g), and propor tionally short in the shaft. The ischia in Rodents are generally long, especially in the Beaver and Jerbow In the latter animal they are directed much outwards, with tuberosities large, much expanded, and everted posteriorly, to give firm attachment to the strong sacro sciatic ligaments. They are placed in a right line with the ilia, the ilio-ischial angle being wanting as in the Carnivora (see fig,. 112. S.).
In the Hare the tuberosities of the ischia are large, and present well-marked lateral pro cesses (e), which are-everted, and rise above the level of' the coccygeal vertebrae. Thepubes are long and slender, and generally join with the ischia in a long median symphysis projecting in a sharp, anterior, vertical, ridge (f); except in the Porcupines, Rats, and Mice, which have a short symphysis pubis. In the Jerboa there is a slight pubic spinous process, very externally placed ; this is better marked in Hares (d), Rabbits, and Beavers. The ilio pubie angle in Hares and Rabbits is shout 120° to 130°. In the Jerboa it is more oblique, 145° ; and still more so in Itats and Mice, 150'.
The sciatic notch is generally long and nar row, especially in the Jerboa; and the obturator foramen very large, particularly in the Beaver. The pelvic cavity and outlets are large and capacious, especially in the Jerboa, in which, by the outward direction of the ischia, the poste rior outlet is much larger than the anterior. In the Beaver also the transverse pelvic diameter is large, and separates widely the hinder ex tremities of these animals, to aid their swim ming habits. In the Capybara however, the pelvis is of little capacity.
In the Guinea-pig it is compressed so much laterally, that the anterior opening is of a triangular shape, with the apex at the pubic symphysis. According to Le Gallois, it measures only 11 millimetres transversely in a full-grown female, while the fcetal head measures 20 millimetres across. Three weeks before labour begins, the elastic liga ment uniting the symphysis pubis becomes thick, soft, and nioveable ; and eight to ten days before, the pubes, turning on the sacro iliac joints as on a hinge, begin to separate rapidly, till the time of parturition, when they admit one or two fingers between them. After accouchment, the symphysis quickly returns to its original condition, and in a few days presents only a little thick ness and mobility, the process being delayed only by ag,e and sickness. Le Gallois found the sacro-iliac syrnphysis also very moveable, so as to allow of the retreat of the sacrum, and the increase of distance between its ex tremity and the pubic symphysis when the fcetal head pressed upon it. In this manner the pelvis of the Guineapig is widened, during labour, in all its diameters. Its young are produced in a very advanced state of de velopment, and are able to run about and eat as soon as they are born.