OR OSSEOUS SYSTEM. 107 



afforded to the broad hoofs, and a broad base for the pon- 

 derous carcase of this powerful quadruped. The carcase 

 being still more ponderous in the elephants, the scapular 

 and pelvic arches, and the whole extremities, are more 

 vertical in their direction, and the scapular and iliac bones 

 are of great breadth. The heavy molar teeth, and the large 

 tusks, and the large proboscis give so much weight to the 

 head, that the neck in these proboscidian animals is very 

 short, and the external surface of the cranium is greatly 

 extended for muscular and ligamentous attachments, with- 

 out adding to the weight of the head, by the vertical cells 

 of the diploe being filled with air admitted through the 

 Eustachian tubes. From the strong attrition to which the 

 molar teeth of the elephants are subjected, they are com- 

 posed of numerous thick transverse plates of enamel en- 

 closing the osseous portions and united together by an en- 

 veloping crusta petrosa, and they are successively worn 

 down to their base, and replaced by new teeth from behind, 

 for eight or nine times during the life of the animals, while 

 the long tusks are renewed but once. The base of the 

 lower jaw projects more than the human. 



In the monotrematous animals the skull is thin, smooth, 

 and diaphanous, and with a lengthened toothless muzzle, as in 

 birds, as we see in the skeleton of the ornithorhyncus (Fig. 

 57 ,) where there are only two horny thin crowns of molar 

 teeth (A,) at the back part, and on each side of the two jaws. 



FIG. 57. 



Their intermaxillary bones converge at their free anterior 

 extremities ; there is a median longitudinal osseous crest in 

 the ornithorhynchus, extending along the interior of the occi- 

 pital and parietal bones, and the occipital foramen (d,) is 

 prolonged upwards, narrow in a vertical direction. The 

 scapula (/,) especially in the ornithorhyncus, is lengthened 



