TEETH OF THE MESOZOIC MAMMALS 31 



4. 



THE ORIGIN OF THE TKITUP-EI;CI:LAI; TYPE SOUGHT (1888) AMONG 



THE MESOZOIC MAMMALIA. 



[Extract from Memoir entitled "The Structure and Classification of the Mesozoic Mammalia." 

 Jonrn. Acad. Nat. S<->. Pfiifa., Vol. IX., No. 2, July, 1888.] 



() If, as now seems probable, the derivation of the mammalian molar 

 from the single reptilian cone can be demonstrated by the comparison of 

 a series of transitional stages between the single cone and the three-cone 

 type, and from the latter to the central tritubercular type, the separate 

 history of each cone can certainly be traced throughout the series in its 

 various degrees of modification, development, and degeneration. The 

 remarkable part played by the tritubercular molar has been unfolded by 

 the discoveries and writings of Cope. It is undoubtedly the ancestral 

 molar type of the Primates, the Carnivora, the Ungulata, the Cheiroptera, 

 the Insectivora, and of several, if not all, of the Marsupialia. For 

 example, we can trace back the quadritubercular btmodont, or parent 

 ungulate type, to the tritnlto'culai- ; this to the type with three cones in 

 line, which we may call the triconn<l<>nt. type, and this in turn to the 

 Tiaplodont l reptilian crown. A nomenclature may be suggested for these 

 cones, with reference to their order of development and primitive position, 

 to keep clearly before the mind their homologies during secondary 

 changes of form and position. The primitive cone may be called the 

 protoconc ; upon the anterior and posterior slopes of which appear, 

 respectively, the paracone and iininrniir. After the tritubercular crown is 

 produced, by the rotation of the lateral cones, inwards in the lower jaw 

 and outwards in the upper jaw, the Jit/poconc, or heel, is developed, giving 

 us the tubercular-sectorial molar. Exclusive of the MnHiliili-rcnlfita and 

 of Stereognathm (Fig. 51, No. 5) this is the most advanced stage of 

 molar development thus far found in the mesozoic period. 



The protocone of Dromatherium (Fig. 3, No. 2) is prominent and con- 

 stant through the molar series while the para- and meta-cones are irregular 

 in size and position, always close to the main cone and in several teeth 

 either splitting into two needle-like cusplets or bifid at the tip. Alto- 

 gether, they are in what appears to be an experimental stage of develop- 

 ment. Microconodon (Fig. 3, No. 1), however, from the same strata, has 

 well defined para- and meta-cones which are widely separated from the 

 main cone, the crown presenting the pure triconodont type. This 

 reoccurs in Amplii.l<'*t<'s (Figs. 4, 5), of the lower Jurassic, and Tricomxlnn 

 (Figs. 4, 7) of the upper Jurassic. In this series we are struck by the 



1 See Cope : "The homologies and origin of the types of molar teeth of the Mammalia 

 Educabilia," Jour. Phila. Ami:/., 1874. 



