426 E. S. GOODRICH. 



It is well known that Osborn (16, 17), Cope (7), and other 

 advocates of the theory assume that the primitive mammalian 

 molar was represented by a simple "reptilian cone/^ the 

 " protoconid," forming the Haplodont type of tooth. The 

 protoconid subsequently acquired a cusp in front and behind 

 ("paraconid" and "metaconid^' respectively), giving the Tri- 

 conodont type. This tooth with three cusps in a line would 

 then have become converted into the Tritubercular type of 

 molar by the movement outwards of the median large proto- 

 conid, and inwards of the anterior paraconid and posterior 

 metaconid. A small posterior " heel '^ then became developed, 

 which subsequently formed an external cone, the '' hypoconid," 

 and an internal cone, the " entoconid," yielding the Tri- 

 tubercular sectorial type of molar seen in most orders of 

 Mammalia. 



Let us now examine the facts. If the primitive mammalian 

 molars were simple cones, we should expect to find a gradual 

 approximation to this condition on comparing the more 

 specialised living mammals with the more primitive, the later 

 fossils with the earlier. But, as a matter of fact, we find on the 

 contrary that these simple molars are only seen in such highly 

 modified forms as the Cetacea, and that no early mammalian 

 fossils whatever have been yet discovered possessing them, 

 whilst multituberculate forms increase in number the lower we 

 search. Dromatherium and Microconodon, animals having 

 teeth with one large and several accessory cusps, presumed to be 

 intermediate between the Haplodont and the Triconodont types, 

 are possibly not mammals at all, but reptiles; at all events, 

 they differ so widely in structure from any known mammal, 

 living or extinct, that they can afford little certain information 

 on this question. The Reptilia themselves cannot be said to 

 support the theory, for the most mammalian of all known 

 reptiles, Galesaurus planiceps, has molars with three 

 distinct cusps. On the other hand, the most reptilian of all 

 living mammals, Ornithorhynchus, has multituberculate teeth. 

 Dealing next with the Triconodont type of molar, we find here 

 again that it does not occur amongst primitive forms, but in 



