550 ORIGIN OF MAMMALS xix. 9 



from other mammals that it is certain that they left the main stock 

 far back in the Mesozoic. Their organization possibly shows us many 

 of the characteristics of mammalian populations at that time. 



Since we are comparing the platypus and echidna chiefly with 

 Mesozoic reptiles we shall deal first with their hard parts, examining 



Fig. 324. Diagram of arrangement of jaw and auditory ossicles. A, in a reptile; 



B, in a mammal. 

 ang. angular; art. articular; cor. coronoid; d. dentary; ex. col. extracolumella; hv. hyoid; 

 inc. incus (quadrate); jon. gonial; mall, malleus (articular); Meek. Meckel's cartilage; 

 pr.fol. processus folianus (goniale); q. quadrate; st. stapes; suran. surangular; tymp. tym- 

 panic (angular). (After Ihle, from Gaupp.) 



Fig. 325. The temporary upper teeth of the duck-billed platypus. 

 (From British Museum Guide.) 



the living animals as if they were fossils. The lower jaw consists of a 

 single dentary bone. The quadrate, articular, and tympanic have 

 entered the ear, but the malleus is large, the incus small, and the 

 stapes elongated (Fig. 324). The tympanic bone forms a partial ring 

 around the tympanum, and the whole apparatus is not enclosed in a 

 bony 'bulla' as it is in modern mammals. Neither animal possesses 

 true teeth in the adult, the platypus having a flattened bill covered 

 with soft skin and used for 'paddling' for the small aquatic animals, 

 especially mussels and snails, on which it lives. Tachyglossus and the 

 related New Guinea form Zaglossus have long 'beaks' for eating 

 ants. However, in the young platypus flattened, ridged teeth are 



