xiii.] MONOTREMATA. 241 



1 8. The hyoid has a small, more or less lozenge-shaped 

 basihyal, broad ceratohyals, the remainder of the anterior 

 cornu usually unossified, and stout somewhat compressed 

 thyrohyals. 



Order MONOTREMATA. Both the animals of this group 

 present very singular modifications of the cranium. 



The cerebral cavity, unlike that of the lower Marsupialia 

 or the Reptiles, with which they have so many structural 

 affinities, is large and hemispherical, flattened below and 

 arched above, and about as broad as long. ' The broad cribri- 

 form 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 com- 

 ponent bones. In both species, the broad occipital region 

 slopes upwards and forwards, and the face is long and much 

 depressed, though of very different shape in each. 



In the Echidna (Fig. 74) the squamosal is large and very 

 compressed, the zygomatic process arising very far forward ; 

 the slender hori/ontal 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 an- 

 terior nares form an oval opening on the upper surface near 

 the apex, bounded entirely by the premaxillaj, 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 (/*/), 

 considerably beyond the glenoid fossa. The narial canals 

 have very little extent vertically, but the true olfactory 

 chambers are large, and provided with complex turbinals, 

 which, in accordance with the horizontal position of the 

 cribriform plate, are mostly placed vertically. 



