xni PHYLUM CHORDATA 521 



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 Echidna. There is a large cotyloid in Ornithorhynchus, 

 larger than the pubis. With the anterior border of the pubes 

 are articulated a pair of large epipubic or " marsupial " bones 

 (Fig. 1127, ep. pb!). The femur has expanded extremities with 

 prominent external and internal trochanters. There is a large 

 ossified patella {■pat.). The fibula (/'6.) has at its proximal end 

 a remarkable compressed process which ossifies from a separate 

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

 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 all the phalanges 

 except the last. 



Skeleton of Metatheria. — In the Marsupials the inferior arch 

 of the atlas (Fig. 1130) is often incompletely ossified, a gap being 

 left in the prepared skeleton; 

 sometimes the gap becomes 

 closed in by the ingrowths of 

 the lateral parts of the arch, 

 sometimes a small separate ossi- 

 fication is developed filling up 

 the opening. In the trunk 

 there are always nineteen verte- 

 brse. The transverse processes pig. iiso.—Atias of Kangaroo. 



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. 1131). Chevron 

 bones are generally present, except in the Koala and the 

 Wombat, 



In the skidl (Figs. 1132-1134) the brain-cavity is relatively 

 small, with the cerebellar fossa entirely behind the cerebral. 

 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- 

 root of the zygoma to form part of the outer wall of the glenoid fossa. 

 The lacrymal foramen is usually on the anterior margin of the 

 orbit, sometimes on the face. The palate usually presents vacuities 

 in its posterior portion. The "pterygoid" (p. 511) is always 

 small. The alisphenoid is large, and forms the anterior boundary 

 of the tympanic cavity; in the Kangaroos (Fig. 1133, ali) it extends 



