242 



THE SKULL. 



[CHAP. 



The pterygoids (Pt] are flattened, horizontal, oval plates, 

 attached to the obliquely truncated postero -external extremi- 

 ties of the palatines, and form part of the floor and the inner 

 wall of the tympanic cavity, an arrangement not met with in 

 any other Mammal. The tympanic (Ty) is a very slender 



ring, incomplete for a small space 

 at the upper and outer end, and 

 does not become ankylosed to 

 the periotic. The latter (Per) is 

 large, has no fossa for the floc- 

 culus, sends out a large expansion 

 (pterotic), forming a portion of 

 the cranial wall between the 

 squamosal, parietal, and occipital, 

 the lower and hinder part at least 

 of which corresponds with the 

 mastoid portion. 



Each ramus of the mandible is 

 a mere slender style, without any 

 ascending portion, and with but 

 rudiments of coronoid process 

 and angle. The condyle is very 



FIG. 74,-Under surface of cranium Small, elongated from before 

 of Echidna (Echidna aculcata), \. K. lr l,iv'ird<; nnd vr>rv mrrmv 



BO basioccipitai; ExO exocci- Dackwarob, ana vtry narrow . 



The hyoid (Fig. 75) has a well- 

 ossified transversely extended, 

 flattened and arched (with the 

 concavity forwards) basihyal (bJi). The anterior cornu has 

 only two ossifications (eh and ch\ apparently the epi- and 

 cerato-hyal, as the upper one has a long ligamentous con- 

 nection with the cranium in the situation of the stylohyal. 

 There are broad, flattened, curved thyrohyals (tli) expanded 

 at their laryngeal extremities. 



pital ; Per periotic ; m malleus 

 Sq squamosal ; Ty tympanic ; Pt 

 pterygoid ; /'/palatine ; MX max- 

 illa ; PMx premaxilla. 



