xiii.] MONO TREMA TA . 24 3 



In the Ornithorhynchus the brain case is smaller than in 

 the Echidna, and rather more depressed ; very broad behind 

 and narrowing anteriorly. The olfactory fossa is compara- 

 tively small. There are well-marked posterior clinoid 

 processes. The falx cerebri is largely ossified, forming a 

 strong median partition to the upper part of the cerebral 

 cavity. The zygoma is compressed, and of considerable 

 vertical depth, and sends up a well-marked postorbital 

 process ; its hinder root arises very far back on the 

 cranium. 



The glenoid fossa is wide and concave transversely. The 

 zygomatic process of the maxilla is widened inferiorly into 



FIG. 75. Lower surface of hyoid of Echidna (Echidna, acnleata). eh epihyal ; 

 ch ceratohyal ; bh basihyal ; th thyrohyal. 



an oblong, concave, roughened surface for the attachment of 

 the horny plate, which takes the place of the molar teeth. 



The face is broad and much flattened. It runs out 

 anteriorly into two diverging processes, each formed by the 

 premaxilla, supported by a pointed process of the nasal on 

 the inner and of the maxilla on the outer side. These bend 

 towards each other at their extremities, but do not meet in 

 the middle line. They support the partly horny, partly 

 membranous beak, which fills up the space between them,' 

 and extends considerably on each side and in front. There 

 is a distinct median dumb-bell-shaped ossification in the 



R 2 



