INTRODUCTION. 



5 



II. Genesis and Evolution of Single Dental Characters. 



The fundamental characters shown in the Lower Oligocene Mesohippus molar and succeeding types are illustrated 

 in Fig. 3. The following single new characters are successively added: 



1. The hypostyle, the bud-outgrowth on the post-cingidum in Mesohippus bairdii. 



2. The crochet, the bud-outgrowth on the metaloph of Mesohippus trigonostylus, Miohippus brachy stylus, M. gidleyi, 

 Parahippus, and all higher equines. Absent in Kalobatippus and Hypohippus. 



3. Degeneration of cingula, last vestiges of internal cingulum being seen in Miohippus, Anchitherium, and Hypohippus. 



4. Expansion of the conules (pi, ml), reduction of the cones (pr, hy), begun in Miohippus, continued in Parahippus. 



5. Selenoid enlargement of the conules (pi, ml) begun in Parahippus, completed in Merychippus. 



6. Forking or bifurcation of the crochet to form the pli crochet and the pli caballin, two of the enamel folds which lie 

 within the prefossette and medivallum respectively, as well seen in Merychippus isonesus primus and M . isonesus secundus. 



pli postfossette 2 

 pli postfossette' 



parastyle 



mesostyle 



Parahippus 



Merychippus 



pli prefossette. 

 -pli hypostyle' 



■pli protoconule 



pli caballin'- 



crochet 



metastyle 



pli postfossette 

 -pli hypostyle' 

 -hypostyle 



Fig. 2. Accessory folds of the equine upper molars. 

 Upper molars of Parahippus and Merychippus, with key to same, showing the exact mode of origin of the accessory 

 folds, or plicas, arising from the primary cones, conules, and lophs. 



7. Forking or bifurcation of the hypostyle to form the pli hypostyle within the postfossette and postvallum, as seen in 

 Merychippus isonesus primus and M. isonesus secundus. 



8. Infolding of the protoconule within the prefossette to form the pli protoconule. 



9. Ptychoid foldings, or plications, of the anterior enamel wall of the metaloph are seen in Parahippus crenidens, P. cognatus 

 (Plates S, 9) and more advanced stages, to form the pli prefossette. 



10. Ptychoid plications of the posterior enamel wall of the metaloph, beginning in Merychippus isonesus primus, M. 

 isonesus secundus and higher stages, to form the pli postfossette. 



11. Reduplication or doubling of the pli caballin, pli crochet, pli hypostyle, observed in progressive species of Mery- 

 chippus, e. g. M. paniensis, M. sphenodus, M. calamarius, M. re publicanus , to form double or triple plications. 



All these secondary enamel foldings and plications are characters primitively confined to the summit, the upper third, 

 or upper half of the crown of the tooth; they may disappear entirely as the crown becomes worn down, as shown in Mery- 

 chippus sejunctus in three stages of wear (Plate 16.1,3). 



The term or prefix pli is taken from the old French term pli caballin, derived from the Latin plica. See Fig. 2. 



crochet forward fold from the metaconule 



pli crochet 1 superior fold of the crochet (Fig. 3) 



pli hypostyle 1 antero-external fold of the hypostyle 



pli caballin 1 fork of the crochet into the medivallum 



pli caballin 2 " " " " reduplicate 



pli prefossette fold of the metaloph into the prefossette 



pli hypostyle 2 duplication of the pli hypostyle 



pli crochet 2 duplication of the superior pli crochet 



pli postfossette 1 fold of the metaloph into the postfossette 



pli postfossette 2 " " " " reduplicate 



pli protoconule fold of the protoconule (pli protoconule, 



Fig. 2) into the prefossette 

 pli crochet 3 superior triplicate fold of the crochet 



