alisphenoid bulla 



eustachian tube and foramen ovole 

 tympanic 



anterior lacerate fenestra 



groove for palatine branch of VII 

 orbitosphenoid porasphenoid 



exoccipital process 



occipital condyle 



.foramen magnum 



Figure 3-6. Details of base of the skull of a 

 (ventral aspect up). 



bone. The occipital bones differ in that the exoccipitals 

 meet broadly above the foramen magnum. A postparietal 



is usually lacking in marsupials; in the opossum, however, 

 it is present and is fused with the supraoccipital. The 

 cranial capsule is relatively smaller, less rounded, and is 

 roofed by the frontals and parietals, which meet along the 

 midline as a sharp sagittal crest. The nasals are narrow 

 anteriorly and much expanded posteriorly. 



Internally (Figure 3-5 C,D) the cranial cavity is relatively 

 smaller, lacking a tentorium and lacking a plate-like dor- 

 sum sellae. The clinoid processes connect from front to 

 back on either side of the sellar depression. There is a large 

 perpendicular plate of the ethmoid fitting into the notch of 

 the vomers. As in the placentals, the turbinals are ossified, 

 and, the internal foramina of the petrosal are the same. 

 There are no frontal or maxillary sinuses. 



The mandible is not unusual, except that the angular 

 process is medially rather than posteriorly directed. The 

 hyoid (Figure 3-1 1 A) is much like that of the rabbit. There 

 is a pair of small plate-like hypohyals, each tapering later- 

 ally to a point at the attachment of the stylohyoid ligament. 

 The hypohyals articulate with a thick body, or copula, 

 which in turn articulates posterolaterally with a large cerato- 

 branchial 1. 



y hypoglossal foramina 



Ijj ^jugular foramen 



exoccipital process 

 fenestra cochleae 



foramen primitivum VII 

 half-grown opossum as seen in a ventrolateral view 



The head skeleton of the opossum differs markedly from 

 those placentals we have examined, but when compared with 

 others, especially the insectivores, it is found to represent only 

 an extreme. The marsupial head skeleton is best character- 

 ized as having an unequal number of incisors above as com- 

 pared with below, as having the jugal contributing to the 

 articular area of the mandible, and as having a basically 

 alisphenoid bulla (a posterior chamber from the petrosal 

 may be added). The fenestrated palate, a confluent optic- 

 orbital fissure, and the lack of a postparietal (the opossum 

 is exceptional in having this bone) are typical but not ab- 

 solutely distinctive. The appearance of a parasphenoid rudi- 

 ment in the young is matched in several placentals (camel, 

 flying lemur). 



Head skeleton of the monofreme 



The monotreme head skeleton can rarely be studied from 

 actual specimens but is worth describing and illustrating. 

 Of the three living genera found in Australia and New- 

 Guinea, two can be compared as representatives of the group: 

 the Duck-billed Platypus, Ormthorhynchus, and the Spiny 

 Anteater (or Echidna), Jachyglossus. The exact details of the 



44 • OSTEOLOGY AND THE MAMMALIAN HEAD SKELETON 



