also a fluid-filled synovial sac associated with the function- 

 ing of this joint. 



Genera/ observations The detailed picture of the mammal 

 skull that has been presented indicates that there is a gen- 

 eral pattern of bones in spite of many distracting minor 

 variations. A review of the Metatheria (or Marsupialia) and 

 the Prototheria (or Monotremata) as contrasted with the 

 Eutheria (or Placentalia) will serve both to clarify the pat- 

 tern and to introduce the nonmammalian type. 



Head skeleton of the marsupial 



The opossum is a readily available example of the mar- 

 supials and a good one since it is considered to be one of the 

 least specialized (or altered) members of the group. At first 

 glance the skull resembles that of a carnivore (Figure 3-5). 

 The premaxilla has five incisors, and the maxilla has a 



large diastema followed by a large canine, three premolar, 

 and four molar teeth (formula: I %, C M, P %, M %). The 

 secondary palate has two large fenestrae on either side of 

 the midline and the posterior margin of the palatine ha.s a 

 transverse ridge on it. The pterygoids are prominent (usu- 

 ally lost in prepared skulls) and they are not fused to the 

 alisphenoid. In the young animal a small splint of bone lies 

 along the midline spanning the basisphenoid-basioccipital 

 joint. This is the parasphenoid (Figure 3-6). 



The alisphenoid is large and has a lateral process, which 

 contributes to the glenoid articular area (or fossa) for the 

 lower jaw. There is a large jugal, whose posterior end also 

 contributes to the glenoid fossa. There is only a small "bulla," 

 formed by the alisphenoid; the bulla does not conceal the 

 petrosal. The tympanic fits into the opening of this bulla 

 but is not fused to any of the surrounding bones. The an- 

 terior process of the malleus (the prearticular bone) is long 

 and overlaps the tympanic, but it is not fused with that 



coronoid process 

 lacrimal frontal 



nasal 



parietal 



optic-orbitol fissure 



,„„„ , L orbitosphenoid 



zygomatic arch »l •_! i 



^ ^ - I , ethmoid foramen 

 ^postparietal and 



supraoccipital 



alisphenoid 



squamosal 



xoccipital process 

 tympanic 



sphenopalatine foramen 

 angular process palatine canal 



petrosal 



fenestra vestibuli 

 postglenoid process 

 transverse canal carotid canol 



mental foramen 



dentary 



-7"^ 



cribriform plate 

 ethmoid turbinals 

 perpendicular plate of ethmoid 

 maxillary turbinal 

 nasal 



frontal 



alisphenoid 



palatine fenestrae. 



carotid canal 

 transverse canal 



parietal postparretal and 



supraoccipital 

 subarcuate fossa 



jugal. 

 .endolymphatic 



foramen 

 jugulor foramen 



hypoglossal foramim 

 condylar foramen 



exoccipital process 



, basioccipitol . . , , 



' glenoid fossa 



postglenoid process 



postglenoid canal 



I t .1 r squamosal 



layers of mastoid process -^ . . _ 



hypoglossal foramina 



palatine 



foramen magnum petrosal 



Figure 3-5. Skull and mandible of the opossum. A, lateral view; B, lateral view of cranium with 

 zygomatic arch cut away; C, ventral view of skull; D, medial view of right half of skull. 



alisphenoid 



foramen 



primifivum 



exoccipifol process 



MAMMALIAN HEAD SKELETON • 43 



