ORDER MONOTREMATA. 



1265 



indeed, quite within the bounds of probability that the family 

 Dromatheriidce mentioned under the head of the latter group 

 may prove to be Prototherians. And in any case it is quite 

 evident that the two existing genera of Monotremes can in no 

 way be looked upon as actual ancestral types. 



Order i. Monotremata. — As already mentioned, both the 

 existing genera of Prototherians are included in this order, which 

 may be provisionally characterised by the production of the cranium 

 into a more or less elongated rostrum supporting a horny beak ; by 

 the absence of teeth in the fully adult animal ; the smoothness of 

 the brain-case ; and the absence of an auditory bulla. The humerus 

 is greatly expanded, and has an entepicondylar foramen ; its whole 

 contour approximating to the corresponding bone of the Anomodont 

 Reptiles (fig. 982). 



Family Ornithorhynchid^e. — In this family the cerebral hemi- 

 spheres are smooth ; the extremity of the muzzle is produced into a 



Fig. 



Ornithorhynchus paradoxus, Australia. Reduced. 



broad beak-like expansion (fig. 1131); teeth are present in the 

 young, and are succeeded by horny plates or cornules (fig. 11 30, b, 

 c) ; and the skin is covered with hair. The feet are webbed. This 



