MOLAR TEETH OF THE MAMMALIA EDUCABILL\. 243 



not modified ; but in the Lophodont types it is materially altered. 

 In Hyracodon, Rhinocerus, Anchiiherium, Oreodon, Cervus, and 

 others, it is flattened, with the long diameter anteroposterior, and 

 is grooved on the inner face, so as to assume the form, as it does 

 the function, of two roots conjoined. In the temporary dentition 

 of Sus, Merychyus, etc., the inner root is divided, so that the 

 superior molar has four distinct roots. 



The proper homologizing of the various forms of dental struct- 

 ure of the Ungulates with each other, and with the primitive 

 types of tubercular teeth, is entirely essential to their intelligent 

 classification, and therefore comprehension of their mode of origin. 

 In order to lay a foundation for this work, I define the four types 

 as follows, giving the subdivisions of the first two in brief, and 

 discussing those of the third and fourth more fully afterward : 

 Division I. Haplodont type ; the crown undivided or simple. 

 a. Crown low, obtuse; Cetacea {Be- 

 luga), Carnivora (Rosmarus). 

 h. Crown elevated, acute ; Cetacea 

 {Delpliinus) ; canine teeth in 

 general. 

 c. Crown truncate ; Edentata {Bra- 

 dypus) ; Rodentia ( Geomys, 

 Dipodomys). 

 Division II. Ptycliodont type ; the crown folded on the sides ; 

 the folds frequently crossing the crown. 

 a. Sides only folded ; Rodentia 

 {Arvicola Castor) ; Edentata 

 {Glyptodon). 

 h. Summit of crown also folded ; 

 Rodentia {Lepus, CMnchilla). 

 Division III. Bunodont type ; crown supporting tubercles. 

 a. Tubercles few, opposite ; Un- 

 gulata, Achmnodon, Dicotyles, 

 Elotherium ; Carnivora, Pro- 

 cyon ; Rodentia, Heliscomys. 

 J). Tubercles few, alternate ; Ihjop- 



7 Fio. 17. — LeptocTicerus. 



sodus. 

 c. Tubercles numerous, irregular ; Mastodon, Phacochcerus. 

 Division IV. Lophodont type ; the summit of the crowns 

 thrown into folds of transverse or longitudinal direction. Higher 

 Ungulates. 



Fig. 15. — Glolicephalus. 



Fig. 16. — Jaeulus hudsonicus. 



