subdivided into dorsal, ventral, and palatine process parts. 

 In the young there are teeth on the maxilla and premaxilla, 

 but in the adult only horny tooth pads mark the positions 

 of these teeth. Of interest is the fact that the prenasal pro- 

 cesses of the premaxillae, which meet at the midline be- 

 tween the external nares, bear an egg tooth for cutting the 

 shell at the time of hatching. Tooth and processes then 

 disappear. 



The secondary palate involves the maxilla and palatine 

 and extends far back into the mouth. The ma.xilla has a 

 strong zygomatic process, which overlaps for a considerable 

 part of its length the process of the squamosal. The jugal 

 is reduced to a small bit of bone, which forms the postor- 

 bital process of the zygomatic arch of the adult when fusion 

 obliterates the sutures. The frontals are very small and ex- 

 tend well down into the orbit. There is no lacrimal; the 

 lacrimal canal opens between the frontal and the ma.xilla. 

 The palatine extends up into the orbit to meet the frontal. 

 Behind the frontal there is a large parietal forming the roof 

 of most of the cranial cavity. The posterior aspect of the 

 cranium is covered by the four occipitals. 



The lateral aspect of the capsule is covered in part by 

 the squamosal, which has a dorsal and a ventral wing be- 

 tween which passes the temporal canal. This canal opens 

 posteriorly as the posttemporal fossa and anteriorly into the 

 temporal fossa. The ventral wing of the squamosal forms 

 the glenoid articular area for the mandible. The squamosal 

 has a strong zygomatic process. 



Medial to the squamosal and forming much of the wall of 

 the brain capsule is a dermal plate, fused with (and in 

 development appearing to grow out from) the petrosal. The 

 petrosal has a posterior mastoid part, with a strong ventral 

 mastoid process, and a ventral vestibular part. The latter 

 contacts the basioccipital medially and the basisphenoid 

 anteromedially. On either side of the nasal passage, the 

 basisphenoid has a strong downgrowth, which sutures ven- 

 trally with the palatine portion of the secondary palate. 

 This basipterygoid extension is usually described as the 

 "alisphenoid," but such an identification seems to be based 

 more on the feeling that it is necessary to have this chara- 

 cteristic mammalian bone in the monotreme than upon its 

 form or embryological development. This case brings up the 

 question of how far homologies can be carried. It seems best 

 to abandon use of alisphenoid, whose dermal component 

 has already been described as forming a part of the petrosal. 

 The pterygoid lies inside of the ventral extension, the 

 basipterygoid process, of the basisphenoid. It is exposed in 

 the orbit, above the palatine and anterior to this process, 

 where it forms the lateral wall of the choanal passage. On 

 the midline and separating the right and left nasal passages 

 is the nearly vertical plate of the ventrally fused vomers. 

 The vomers are exposed posteriorly, where they extend back 

 to slightly overlap the basioccipital. 



At the posterior end of the secondary palate and extend- 

 ing out horizontally to either side is a plate of bone that re- 



mains free in the adult skull and is usually lost in prepa- 

 ration. The exact nature of this plate is not clear, but it is 

 here identified as ectopterygoid. 



Between the pterygoids, and anterior to the basisphenoid, 

 is the midline fused mass of the orbitosphenoids. It extends 

 upward and backward on either side as a large cranial-wall 

 wing. 



The tympanic bone is a semi-ring lying horizontally in 

 the floor of the tympanic cavity. It is fused to the anterior 

 process of the malleus, and this is fused or tightly bound to 

 the incus. The stapes is columelliform (not penetrated by a 

 stapedial foramen) with a round foot plate fitted into the 

 fenestra vestibuli. 



An internal view of a half skull cut on the median sagit- 

 tal plane (Figure 3-8 D) is comparable to what we have seen 

 in the placental, but the turbinals are quite small. There is 

 a small perpendicular plate of the ethmoid fused to the 

 anterodorsal end of the orbitosphenoid. The transverse 

 ethmoid ossification is perforated to either side by an ol- 

 factory foramen but is not cribriform. 



The dorsum sellae has strong posterior clinoid processes 

 extending forward on either side of the sella. There is no 

 tentorium but there is a midline sagittal ossification in the 

 falx cerebrae, the connective tissue sheet extending down 

 from the cranial roof between the cerebral lobes of the brain. 

 There is a large subarcuate fossa. 



The mandible is highly specialized and has large posterior 

 fossae on the outer and inner surfaces behind external and 

 internal processes. The latter can be compared with the 

 angular process. There is a large mandibular foramen on the 

 inner aspect and a large foramen on the outer surface at 

 the vertical of the anterior margin of the horny tooth plates. 

 The anterior end is flattened dorsoventrally with a large 

 foramen and distal groove on its upper surface and a some- 

 what more distal but corresponding foramen and groove on 

 the lower surface. The two rami of the mandible are joined 

 well back from their spatulate tips. 



The hyoid appartus (Figure 3-1 1 B,C) is like that of the 

 dog or cat, diff'ering in that the hyoid arch has two ossified 

 segments, the hypohyal and ceratohyal. The body is a 

 transverse piece to which a fairly large ceratobranchial I is 

 attached. This ceratobranchial is closely bound to the 

 margin of the thyroid cartilage. Behind the ceratobranchial 

 I is a pair of ossicles associated with the thyroid cartilage 

 which represent the ceratobranchials II, and behind these 

 another pair more closely joined to the thyroid cartilage, 

 the ceratobranchials III. 



Tachyglossus, the Echidna Tachyglossus has a long, toothless 

 snout, and the secondary palate extends far back, the 

 palatines ending at medial but well-separated points (Figure 

 3-9). These features are related to its ant-eating habit. The 

 palatines bear the quite large ectopterygoids on their 

 posterolateral margins. 



The pterygoids lie along the dorsolateral wall of the 



46 . OSTEOLOGY AND THE MAMMALIAN HEAD SKELETON 



