XIII PHYLUM CHORDATA 489 



together with the posterior portion of the inner surface, is the 

 subscapular fossa. 



The humerus is of remarkable shape, with greatly expanded 

 extremities, especially in the Echidna, and prominent tuberosities 

 and condyles. In the carpus the scaphoid and lunar are united : 

 there is no separate centrale. There is a radial sesamoid, and two 

 very large palmar sesamoids, which are sometimes united. 



In the pelvis there is a very long] symphysis in which pubes 

 and ischia take an almost equal share. The acetabulum is per- 

 forated in both genera. With the anterior border of the pubes 

 are articulated a pair of large epi-pubic or "marsupial" bones 

 (Fig. 1059, ep. pb.). The femur has expanded extremities with 

 prominent external and internal trochanters. There is a large 

 ossified patella (pat.). The fibula (fb.) has at its proximal end 

 a remarkable compressed process which ossifies from a separate 

 centre, and greatly resembles the olecranon of the ulna. In the 

 tarsus there are the usual bones. In the Platypus the astragalus 

 and calcaneum are firmly united, and an accessory ossification 

 (ace. tars.} on the inner side in the male bears the tarsal spur. 

 The metatarsals are short and broad : as are the phalanges, except 

 the last. 



Skeleton of Metatheria. In the Marsupials the inferior 

 ,rch of the atlas (Fig. 1061) is often incompletely ossified, a gap 

 being left in the prepared 

 skeleton: sometimes the gap 

 becomes closed in by the in- 



B/ 



growth of the lateral parts of 

 the arch, sometimes a small 

 separate ossification is de- 

 veloped filling up the opening. 

 In the trunk there are always 

 nineteen vertebrae. The trans- Fic; - io<.u.-Atias of Kangaroo. 



verse processes of the thoracic 



vertebrae are always well-developed, and the ribs articulate with 

 them as well as with the bodies. Prominent metapophyses 

 and anapophyses are developed ; these are largest in the lumbar 

 region. Only one sacral vertebra is present in most Marsupials : 

 in some a second is ankylosed with it. The caudal region 

 varies greatly in length. It is short in the Koala and the 

 Wombat, long in the Opossums, Dasyures, Phalangers and 

 Kangaroos (Fig. 1061). Chevron bones are generally present, 

 except in the Koala and the Wombat. 



In the skull (Figs. 1063-1065) the brain cavity is relatively 

 small, with the cerebellar fossa entirely behind the ceiebral. 

 The pituitary fossa is not distinct, and there are no clinoid pro- 

 cesses. The zygoma is complete, but the orbit is not completely 

 bounded by bone behind. The jugal extends beneath the squamosal 



