XIII.] MONO TREMA TA. 2 1 9 



the cranium in the situation of the stylohyal. There are 

 broad, flattened, curved thyrohyals (///), expanded at their 

 laryngeal extremities. 



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 pro- 

 cesses. 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 \vell-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 

 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 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, w^hich fills up the space between them, 

 and extends considerably on each side and in front. There 

 is a distinct median ossification in the triangular interval 

 between the diverging premaxillary bars, placed in, or in 

 front of, the anterior extremity of the mesethmoid cartilage. 

 and apparently corresponding to the so-called '• prenasal ' 

 of the Pig. The infraorbital foramen is very large, cor- 

 responding to the large size of the nerves distributed to the 

 sensitive sides of the beak. The periotic has a wide and 

 deep floccular fossa. 



