CHAPTER XXIII. 



GENERAL ACCOUNT OF THE SKELETON IN 

 MAMMALIA (CONTINUED). 



THE SKULL AND APPENDICULAK SKELETON. 



THE SKULL. 



MONOTREMATA. In both genera the cranium is thin -walled, 

 has a fairly large cavity, and is very smooth and rounded ex- 

 ternally. The sutures between many of the bones early become 

 obliterated in a manner comparable to that in birds, and the 

 facial portion of the skull is much prolonged. 



In Echidna the face is drawn out into a gradually tapering 

 rostrum, formed mainly by the premaxillae, maxillae and nasals. 

 The zygomatic arch is very weak, and the palate extends very 

 far back. The tympanic forms a slender ring. The mandible 

 is extremely slight, with no ascending portion, and but slight 

 traces of the coronoid process and angle. The hyoid has a 

 wide basi-hyal and stout thyro-hyals, while the anterior cornua 

 are slender, and include ossified epi-hyals and cerato-hyals. 



In Ornithorhynchus the zygomatic arch is much stouter 

 than in Echidna. The face is produced into a wide beak, 

 mainly supported by the premaxillae, between whose diverging 

 anterior ends there is a dumb-bell-shaped bone. The maxillae 



