ORDINAL TYPES OF MOLAKS: CARXIYOKA 



139 



In i / innings, so that if we did not know the intermediate history \\- 

 would be entirely misled. This results from the fact as noted above, 

 that, in the Creodonta, teeth becoming upper sectorials are chiefly the 

 molars, whereas in the Fissipeclia they are invariably the fourth upper 

 premolars ; although the initial pattern of the upper molars and of the 

 upper premolars is different, the crown being composed of cusps some 

 of which at least are not homologous with each other, the result of 

 adaptation is to make these two teeth appear to be entirely similar. 

 (Series I., II., Fig. 96.) 



protocam liy/iocon? 



protocone 



e- - f^jf ^ V 



* 



protocone 



FIG. 96. Convergent Evolution of G'aruassials in Creodonta and Fis.sipedia. Arranged by Dr. 

 W. D. Matthew from specimens in the American Museum of Natural History. These series 

 represent morphological but not direct evolutionary sequences. 



I. (Upper row). Creodonta. First upper true molar of the right side evolving from the tri- 

 tubercular into the carnassial type in the Hyienodontidse. 



A 1 . Deltutherium of the Basal Eocene. Generalized tritubercular molar with three primary 

 cusps (pf., pa., i/ii.), a rudimentary hypocone, and two external styles (/'-*., mts.). 



A' 1 . Sinopn op<stl<i>tn,,,a of the Lower Eocene. Forward shifting of the protocone (/'.), back- 

 ward prolongation and cutting shape of the metastyle (nits.'), reduction of the parastyle (y<.<.). 



A3. Sinopa trhitice of the Middle Eocene. The progressive changes described under A- more 

 strongly accentuated; also incipient reduction of the protocone (pr.), and its approach toward 

 the paracone (pu.), but especially the approximation of the paracoue to the metacone. 



A*. Ptfi-oilon of the Lower Oligocene. Still further accentuation of the above tendencies, 

 namely, approximation of the metacone to the paracone (me., pa.), reduction of metacone (/, .), 

 enlargement of metastyle (mtu.), reduction of parastyle (pas.), auteroversion of protocone (pr.) 

 and its approximation to the paracone. 



A 5 , lliiif.niii,!,! of the Middle Oligocene, representing the final stage. Protocone (pr.) reduced 

 to a mi- re- ciiisulum, paracone and metacone (pa.., me.) completely confluent, metastyle (;?.<.) 

 greatly elevated, laterally compressed, giving us a cutting blade laterally compressed, composed 

 mainly of two elements, namely, jiiii-in'une+iiietaeoit.i:, and metastyle. 



II. (Lower row). Fissipedia. Superior fui'i-t/i premolnr evolving from tritubercular into 

 carnassial form in Palseouictidif , Miacidse, Canidse. 



B 1 . Palaonicti*. A Creodont of the Lower Eocene. Generalized fourth premolar type, with 

 protocone, deuterocone, and cusps corresponding to parastyle and metastyle. 



JS2. Did>/mictis of the Basal Eocene. We note the forward shifting of the deuterocone, the 

 reduction of the parastyle, the enlargement of the inutastyle. 



'': Ditplictiiuis, a dog of the Lower Oligocene. Forward shifting of the deuterocone, still 

 further reduction of the parastyle, elevation of the protocone and of the metastyle. 



B*. Cuiiis, a recent dog, showing the vestigial deuterocone and parastyle, and the subequal 

 metastyle and protocone. 



