. Chordate morphology. Morphology (Animals); Chordata. 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 ro
. Chordate morphology. Morphology (Animals); Chordata. 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 3-11 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
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