Having traced the Mammalian pelvis to a form presenting somewhat of the reptile type in the Monotremes, we may now recur back to an order of animals which, froin their general organisation, are connected closely to the order of primates, and are usually placed much higher in the animal scale than the position here assigned to them.
These are the Sloths or Tardigrades, vihich form the connecting link between the Simile and .Edentata proper. Their pelvic peculiarities, however, ally them more closely to the Birds. The most.striking of these is the ossification of the ilia and ischia to the broad sacrum, by transformation of the sacro-iliac and sacro sciatic ligaments. We have already noticed an exceptional example of this coalescence in the Ruminants, in the itleminna or pigmy Chevrotain. But the pelvis of the Edentata also presents a diminution of the yubic sym physis, and the absence of the Ischia from this junction, a separation which is carried still further in the Inseetivora and Cheiroptera. The increasing obliquity of the pubes also indicates an approach to the Bird type.
The climbing habits of the Sloths produce a habitual vertical position of the trunk, re quiring for the support of the abdominal viscera large open pelves.
In the Ai (Bradous tridaetylas) the pelvis (fig. 100.) is remarkably slender, expanded, shallow and horizontal in direction, the pelvic openings being very large and round, and the antero-posterior diameters little larger than the transverse. The ankylosis of the innominate bones to the sacrutn in these animals gives a great firmness to the support of the otherwise feeble hinder extremities, and with the great distance of separation of the acetabula, which are small and shallow, assists to a considerable degree their climbing and holding powers, and to produce that slowness and awkwardness of motion which has given them the name of Tardigrades.
The sacrum is large, both in length and breadth, very flat, with large, open foramina, and presenting a flattened surface in place of the posterior spines and tuberosities. It is composed of five vertebrm, of which the three upper (e) as well as the last lumbar (g) are ankylosed to the ilia (b). The union of the last lumbar seems to result from an extension of ossification in the ilio-lutnbar ligament (i), and contributes much to increase the steadiness of the spinal column on the pelvis. The coccyx is triangular, little curved, broad and short, and is composed of six pieces. In some species it is prolonged into a tal The ilia are short and slender, with much expanded wings, having an anterior concavity and a plane surface posteriorly. They are
ossified to the sacrum at an early period. The ischia are short and slender, and united to the last sacral vertebra, and more slightly to the two above it, by ossification of the great sacro-sciatic ligament (a), which gives to the angle of the bone an expanded appear ance, and encloses a round, wide saero-seiatie foramen (d), above and behind the cotyloid cavity. The tuberosities arc small, and the inferior rami (f) are long and slender, enclosing with the pubis a very large obturator foramen, having its long diameter from side to side, and do 7101 join in symphysis. The pubes (h) are long and slender, their rarni united in a V shape, with the angles meeting to form a very short symphysis (c), which is sometimes ossified, and presenting a very slight ilio-pectineal spine (i). The lumbo-iliae angle in the adult Bradepus is about 145°, and the ilio-pubie about 155°, being only about 25° from a right line as in the human pelvis. The ilio-ischial angle also approaches the human standard in being di minished to 135°.
This diminution of the ilio-ischial angle is still more remarkably shown in the My Won and Megatherium fossil gigantic Sloths, which approach more closely to Man in this respect than any other Mammalian.
The osseous system of the fossil Illylodon robustus closely resembles that of the Sloths, differing from them-, however, by presenting a continued sacral crest, and more expanded ilia (fig. 101.). According to Professor Owen, in his valuable monograph on the specimen in the Hunterian 'Museum, the sacrum really con sists of seven vertebrm, but by ankylosis with the three lumbar and last dorsal includes eleven vertebrm, and forms one strong and con tinuous bony mass along the whole lumbar re gion (a). Its total length is 2 feet 4 inches, and it gradually increases in breadth to the sacro-iliac union (e), which is formed by the first, second, and third true sacral vertehrm, and there presents its greatest breadth. It then contracts slightly, and, at the sixth and last, ex pands again to join the ischia (d). It is firmly united by ankylosis both to the ilia and ischia. Its anterior surface is curved both laterally and vertically. The spinal canal is very wide, and the foramina passing from it mark the primary vertebral divisions. The whole of the eleven spinous processes of the ankylosed vertebrte form a remarkable curved crest pos teriorly (g). There are twenty-one caudal ver tebrm, which doubtless, in the living animal, contributed to support the body by applica tion to the limbs of the trees upon which it climbed, and were strongly supported by the sacro-sciatic ossification.