VI.] MONO TREMA TA. 71 



hypapophyses, but there are two single median chevron 

 bones near the middle of the tail, more like the lumbar 

 subvertebral ossicles of the Mole than ordinary chevron 

 bones. The Ornithorhynchus has 20 or 21 caudal vertebrae, 

 with wide transverse processes, a single median hypapo- 

 physis, and no chevron bones. 



