xix. 9 



CUSPS OF TEETH 



549 



There have been various theories about how the triangular plan was 

 reached. The original tritubercular theory supposed that there was 

 a 'rotation' into a triangular position from the three cusps in line of a 

 triconodont. Even if this could be 

 shown to have happened in a series of 

 fossil teeth, we should still require a 

 knowledge of the change of morpho- 

 genetic process by which the 'rota- 

 tion' was produced. There have also 

 been attempts to explain the many- 

 cusped mammalian tooth as due to 

 the 'fusion' of a number of reptilian 

 tooth germs, either those making one 

 series on the gum or the teeth of 

 successive series. It is plausible that 

 changes in relative time and/or place 

 of tooth development could occur in 

 this way, leading to a partial 'fusion' 

 and the production of many-cusped 

 structures. At present there is too 



little information to decide how the reptilian became changed into 

 the mammalian tooth, but there can be little doubt that the tritu- 

 bercular theory shows us approximately the nature of the earliest 

 mammalian cusp patterns. Indeed it was originally put forward 

 because nearly all the Eocene representatives of the various mamma- 

 lian orders showed signs of a triangular cusp pattern. 



pad. met d. 



•^ LinguaL 



anterior 



Fig. 321. Arrangement of occlusion 



of cusps of tritubercular teeth. Upper 



molars continuous outline, lower 



dotted. 



met. metacone ; metd. metaconid ; pa. para- 

 cone; pad. paraconid; pr. protocone; 

 prd. protoconid; t. talonid (heel). (After 

 Osborn.) 



Fig. 322. Duck-billed platypus 

 (Ornithorhynchus). (From photographs.) 



Fig. 323. Five-toed echidna 

 (Tachyglossus). (From photographs.) 



9. Egg-laying mammals. Subclass Prototheria (Monotremata) 



The duck-billed platypus (Ornithorhynchus) and spiny ant-eater, 

 usually called Echidna but strictly Tachyglossus (Figs. 322 and 323), are 

 Australasian mammals, basically similar to each other, but so different 



