506 ZOOLOGY 



SECT. 



(pat.). The fibula (/&.) has at its proximal end a remarkable 

 compressed process which ossifies from a separate centre, and 

 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 all the phalanges except the last. 



Skeleton of Metatheria. In the Marsupials the inferior arch 

 of the atlas (Fig. 1142) 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 

 FIG. 1142. Atlas of Kangaroo. are always nineteen vertebrae. 



The transverse 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 Kan- 

 garoos (Fig. 1143). Chevron bones are generally present, except in 

 the Koala and the Wombat. 



In the skull (Figs. 1144-1146) 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 processes. 

 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 is always small. The 

 alisphenoid is large, and forms the anterior boundary of the tym- 

 panic cavity ; in the Kangaroos (Fig. 1145, all) it extends backwards 

 so as to join the paroccipital process, which is greatly elongated. 

 When an auditory bulla is developed, it is formed by this bone, 

 the tympanic being always small, and never ankylosed to neigh- 

 bouring bones. The internal carotid artery perforates the basi- 

 sphenoid. The optic foramen is not separate from the sphenoidal 

 fissure. In all except Tarsipes the angle of the mandible sends 

 inwards a remarkable process (ang.), and is said to be inflected. 



In the pectoral arch of the Marsupials the coracoid process is, 

 as usual, developed from a special bony centre, and a distinct 



