108 ORGANS OF SUPPORT, 



and curved backwards, like that of a bird ; and, as also in 

 that oviparous class, the large coracoid bones reach and 

 unite with the sternum, the clavicles (m,) meet and are 

 anchylosed together in front, and the sternal appendices are 

 ossified. The ribs encompass a large proportion of the 

 trunk, and long marsupial bones (1,) are extended forwards 

 from the margin of the pubic bones. The long arms and 

 legs, and the extended feet of the ornithorhyncus suit it for 

 its aquatic life, while the stronger extremities and short feet 

 of the echidna are suited for digging in the ground. In 

 many of the edentata the upper and lower jaws are long, 

 narrow, curved, and toothless, as in birds, and the trunk, as 

 in the armadillos, is surrounded with very broad ribs. The 

 pubic bones also are often lengthened backwards, and meet 

 at a very narrow symphesis, and there is generally a sacro- 

 sciatic foramen, as we find in birds, in place of the ordinary 

 sacro-sciatic notch of quadrupeds. In the sloths there are 

 false ribs anterior to the true ribs, as well as behind them, 

 as we observe in most of the oviparous vertebrata, and the 

 zygomatic arch is open in some of the ant-eaters, as the 

 myrmecophaga jubata. From the longitudinal movement of 

 the lower jaw in the rodentia, its condyles are extended 

 longitudinally, and the layers of enamel are disposed trans- 

 versely in the molar teeth. Their two chisel-shaped incisors 

 above and below are kept sharp by means of the thin layer 

 of very dense enamel which coats their anterior surface, and 

 the broad crowns of their molares are kept rough by the 

 unequal densities of the layers of enamel and of osseous 

 substance which compose them. As the cerebral hemis- 

 pheres are destitute of convolutions, the surfaces of the 

 skull are thin, smooth, and often diaphanous, as in birds, 

 and the squamous portion of the temporal bone generally 

 remains long separate from the other elements of that bone. 

 The mastoid bone generally forms a large bulla communi- 

 cating with the tympanum, as in the carnivorous quadrupeds, 

 and the orbit is here also continuous with the temporal 

 fossa. The intermaxillary and the nasal bones are of great 

 size, the zygomatic arch has its convexity directed down- 

 wards, and the palatine holes are of great size, as in birds. 

 The clavicle is sometimes complete and strong, and in many 

 it is developed only in its central part ; the sacrum and the 

 iliac bones are long, and the pelvis is extended backwards, 



