SKULL — MAMMALS 205 



by cartilage. The choana? are posterior, the paramastoid process is rudimentary 

 or absent; the lower jaw is reduced and its angle, coronoid process and condyle 

 are little prominent. 



Marsupialia have skulls which vary considerably in shape with food and 

 habits, these afifecting the teeth more than other parts. The sutures of the 

 cranium largely persist, the four occipitalia often being separate through life. 

 Orbit and temporal fossa are widely connected, often with no sign of postorbital 

 processes. The zygoma is complete, the zygomatic bone extending into the 

 glenoid fossa. The nasals are well developed ; there is no separate optic foramen, 

 but the foramen rotundum is present. The lacrimal foramen is in the facial part 

 of the lacrimal bone or just inside the orbit. The semicircular tympanic does 

 not fuse with other bones, it and the alisphenoid forming the tympanic wall, the 

 latter sometimes forming a bulla. 



Fig. 215. — Skull of opossum, Didelphys virginiana. 



There are usually vacuities, sometimes maxillary, sometimes palatine, in the 

 hard palate. The paramastoid process is large in kangaroos, smaller elsewhere. 

 The internal carotid artery pierces the basisphenoid as in Monotremes. The 

 lower jaw, except in Tarsipcs, has a high coronoid process and an inflected angle. 

 The hyoid body is lozenge-shaped, the ceratohyals broad. 



IxsECTWORA. — The skull is usually elongate, especially the facial part, and 

 some genera have the foramen magnum nearly vertical. Cranial sutures are 

 nearly obliterated in Talpids, Sorex and some others. Orbit and temporal fossa 

 are usually confluent {Talpa and some others excepted), but often a postorbital 

 process from the frontal partially separates them. The facial part of the 

 lacrimal is large, this often containing the foramen. Many genera have optic 

 foramen and orbital fissure united, the foramen rotundum sometimes being 

 included. Ah- and basisphenoid occasionally contribute to the wall of the 

 tympanic cavity and some genera have a tympanic bulla. The petrosal is 

 sometimes only loosely connected with the rest of the cranium. A zygomatic 

 bone is lacking, unless it be fused with the maxilla, which, in Macroscelides, 

 extends back to the mandibular fossa. Some {Erinaceus, Talpids, etc.) have 

 vacuities in the hard palate, and the vomer in a few genera has broad postero- 

 lateral wings. The halves of the lower jaw are seldom ankylosed, the well- 

 developed angular process is rarely inflected and the condyle is a transverse 

 roller. 



