46 EVOLUTION OF MAMMALIAN MOLAR TEETH 



development of the secondary cusps is subordinated, and the metaconid is 

 almost invariably suppressed. 1 



Ada-pis and Anaptomorphus are examples of sub-types a, c, associated; 

 i'<>r it frequently happens that the paraconid atrophies without a complete 

 enlargement of the hypocone. A study of the diagram demonstrates, 

 however, that the association of sub-types & and c is impossible. The 

 recent monkeys [Primates] Tarsius and Loris afford a good illustra- 

 tion of the association of tritubercular, quinquetubercular, sexitubercular, 

 and quadritubercular molars. 



The subsequent evolution of the molars in different orders was 

 variously characterized, first, by loss of the primary cusps, e.g. the meta- 

 conid in the Carnivora, the paraconid in the Ungulata. Second, by the 

 loss of some of the secondary cusps, e.g. the proto- and meta-conules in 

 the Artiodactyla. 2 '' Third, by the metamorphosis in the form of the cusps. 

 This subject has been fully treated by Kutimeyer, Kowalevsky, Cope, 

 Schlosser, and others. 



The Relation of Trituberculy to the Persistence of Mammalian Phyla 

 The above table shows somewhat indefinitely, but none the less positively, 

 the general progression of the Mammalia to and from the primitive 

 tritubercular type. As already stated, even with our present very 

 limited knowledge, certain stages appear to have been characteristic of 

 certain periods, as follows : the triconodont in the lower Jurassic ; the 

 primitive tritubercular and tubercular sectorial in the upper Jurassic : 

 the secodont and bunodont sub-types of trituberculy, predominated in the 

 Puerco ; in the Bridger, the Perissodactyl ungulates had mostly passed 

 beyond into the lophodont.and symborodont types, and the Artiodactyls 

 were approximately in the stage of sub- type c ; but the Lemuroidea, 

 Creodonta, Insectivora, etc., were, almost without exception, tritubercular. 



There can be little doubt that, parallel with the tritubercular forms, 

 in each period, there were aberrant or degenerate types, but it is difficult 

 to determine which these are. Many Mesozoic types which the writer 

 formerly considered aberrant, have now proven to be tritubercular. 3 The 

 upper Jurassic genera included under the Diplocynodontida? (see Marsh, 

 Amer. Jutmi. Sc., April, 1887, p. 338) are apparently aberrant. There 



1 See Cope, "Origin of the specialized Teeth of the Carnivora," Am. Naturalist, 

 March, 1879. 



2 Schlosser, " Beitrage zur Kenntniss der Stammgeschichte der Hufthiere," Morph. 

 Jahrh., 1886, p. 123, has especially drawn attention to the probability that the Artiodactyla 

 were derived from sexitubercular forms. 



*[Many Artiodactyl families, e.if. Trigonolestida?, Leptochoaridre, Dichobunidse, Anthraco- 

 theriidaa, Elotheriida;, retain the protoconule. The loss of the metaconule is only apparent, 

 for it is represented in the enlarged postero-internal cusp, analogous in position to a 

 cingulum-hypocone, in the molars of most Artiodactyla. ED.] 



3 See "Additional Observations upon the Structure and Classification of the Mesozoic 

 Mammalia," Proc. Phila. Acad., Nov., 1888. 



