structure and development of the head skeletons of these two 

 egg-laying mammals are not clearly understood, but a provi- 

 sional account can be given. From their general form and 

 from the fusion of the bones throughout their skulls, it can 

 be assumed that these head skeletons will reflect many 



specializations. Because of the fusion of parts, their anatomy 

 must be studied from pouch or nest young. 



OrnHhorhynchus, fhe Platypus The bill is made up of 

 nasals, maxilla, and premaxilla (Figure 3-7). The latter is 



nasal 



maxilla 



premaxilla 



dermal petrosal 

 orbltosphenoid 1 ^-r-^ — ^:7~~~^ parietal 



frontal 

 i"g°l-^--^^^ \ -T >~---,-;- • .^,^-^_/\^squamosal 



,supraoccipital 



exoccipital 



temporal canal 



parietal 



orbltosphenoid 



dermal petrosal 



anterior lacerate fissure 



orbital fissure 



tooth 



palatine processes 

 of premaxilla 



crista parotica or 



paroccipital process 



hyoid process 



basisphenoid 

 jugular foramen 



mtraorbiti 



anterior palatine foramen 



foramen rotundum 



stapes 



alar canal 



foramen ovale 



Figure 3-7. Skull and mandible of the Platypus (Ornithorhynchus). A, lateral view of skull and 

 mandible; B, dorsal view of skull; C, ventral view of skull. 



MAMMALIAN HEAD SKELETON • 45 



