12 



ON THE HOMOLOGIES AND ORIGIN OF THE 



culata) in the number of their digits (4, 5), and in which the transverse and longi- 

 tudinal crests of the molar teeth are broken up into tubercles more or less connected, 

 either type of dentition [^. e. Proboscidian or Perissodactyle] being derived according 

 as such tubercles are expanded in the transverse or longitudinal directions."* 



The proper homologizing of the various forms of dental structure of the Ungu- 

 lates 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 afterwards. 

 Fig. 1. Division I. Haplodont type ; the crown undivided or simple. 



a. Crown low, obtuse ; Cetacea {Beluga), Carnivora (Rosmarus). 



h. Crown elevated, acute ; Cetacea {Delphinus) ; canine teeth in gen- 

 eral. 



c. Crown truncate; Edentata {Bradypus); Rodentia {Geoinys Bipodo- 



mys). 

 Division II. Ptychodont type ; the crown folded on the sides ; the folds fre- 

 quently crossing; the crown. 



Fig. 2. ^ J n 



^^Si,,^^gpp^gm^ a. Sides only folded ; Rodentia (Arvicola Castor) ; Edentata 



{Glyptodon). 

 b. Summit of crown also folded; Rodentia {Lepus, Chinchilla). 

 Division III. Bunodont type; crown supporting tubercles. 

 Pig- 3- a. Tubercles few opposite; Ungidata, Achcenodon,^ Dicotyles, Elo- 



theriimi ; Carnivora, Procyon ; Rodentia, Heliscomys. 

 fl'tr' '.'^ h. Tubercles few alternate ; Hyopsodus. 



c. Tubercles numerous, irregular ; Mastodon, Phacochoerus. 



Leptockoerus. 



Division IV. Lophodont type ; the summit of the crowns thrown into folds of 



Jaculus Tiiidsonicus. 



f'^ffiyvi,':, ^ 



Fig. 4. 



Rhinoceros. 



transverse or longitudinal direction. 



This division embraces the many types observed in the 

 Ungulates, some Eodents, and possibly Carnivora. Inas- 

 much as the teeth of the maxillary and dentary (mandi- 

 bular) bones do not always conform to the same type (e. g. 

 Symborodon, Equus), it will be necessary to consider them 

 separately. Besides the diff'erence in type, they differ in 

 their relative development in width in the more special- 



* On the primitive types of Mammalia Educabilia, published by E. D. Cope, May 6th, 1873. 

 t See Cope, Palaeoutological Bulletin, No. 17, p. 2. 



