II.] 



MONOTREMATA. 



217 



structural affinities, is large and hemispherical, flattened 

 below and arched above, and about as broad as long. The 

 broad cribriform plate of the ethmoid is nearly horizontal. 

 The walls are very thin, and smoothly rounded externally, 

 and the sutures become completely obliterated in adult 

 skulls, so that it is very difficult to trace out the boundaries 

 of the component bones. In both species, the broad occi- 

 pital region slopes upwards and forwards, and the face is long 

 and much depressed, though of very different shape in each. 

 In the Echidna (Fig. 70) the squamosal is large and very 

 compressed, the zygomatic process arising very far forward ; 

 the slender horizontal zygoma being completed by a styliform 

 malar, confluent with the maxilla. 

 The face is produced into a long 

 tapering rostrum, rounded above 

 from side to side, and concave 

 below in the same direction. 

 The anterior nares form an oval 

 opening on the upper surface near 

 the apex, bounded entirely by the 

 premaxillae, for the nasals do not 

 appear to reach so far forwards. 

 The alveolar borders are narrow 

 and rounded, without trace of 

 teeth. The palate is produced 

 backwards, by very large palatine 

 bones (PI), considerably beyond 

 the glenoid fossa. The narial 

 canals have very little extent ver- 

 tically, but the true Olfactory Fig. 70.— Under surface of cranium 

 , ... of Echidna {Echidna hystrix), f . 



chambers are large, and provided bo basioccipitai ; Exo exocci- 



• , , 1 • 1 , • , pital; Per perintic ; m malleus; 



With Complex tUrbinals, Which, in sq squamosal ; Ty tympanic ; Pt 



•1 . , - , . pterygoid ; PI palatine ; Mx max- 



accordance with the horizontal ilia ; pm x premaxiiia. 



