TYPES OF MOLAR TEETH OF MAMMALIA EDUCABILIA. 77 



Loxolophodonts. 



Alternate cusps connected by two cross ridges from the outside forwards, and 

 one from the inside forwards ; from which result an oblique posterior cross ridge, 

 and a V opening inwards. 



Here are Eohasileus and Bathmodon ; the last molar of the latter having the 

 anterior ridge of the V quite low. 



3. Comparison of the opposing series. 

 In review, the above types of molar dentition may be classified as follows, with 

 reference to the amount or complication of the modification of the tubercular type. 

 The orders of JJngulata with which they correspond, are also given. 



a. Both inner and outer tubercles crescentoid, 



f Ruminantia. 

 Selenodonts. \ {Anoplotherium, Orendon, Hyopotamus). 



i. Peris so dactyla {Equidm). 



(3. External tubercles only crescentoid; the inner transverse or tubercular. 

 Tapirodonts. ^ 



Palseotheriodonts. >- Perissodactyla in gen. 



Symborodonts. J 



y. Neither kind of tubercles crescentoid, but united in pairs. 

 Trichecodonts. 1 



Bathmodonts. |^ Prohoscidia. 



Loxolophodonts. j 



It may be added that the groups arranged under y are the only ones in which 

 the types of crests of the superior and inferior molars are fundamentally simple 

 and alike. Thus in the group a, tubercles of both upper and lower series are 

 modified independently to produce the type; in group /?, the tubercles of the upper 

 series are modified independently of each other, while those of the inferior series 

 unite, in order to produce the result; in division y the tubercles of both jaws 

 unite entirely across the crown, without any distinction between those of the outer 

 and inner sides. Thus the molar type of dentition of the Proboscidians is the 

 most generalized among the Ungulates, resembling in this respect the type of con- 

 struction of the feet. 



III. The origin of the types of Lophodont dentition. 

 The four types of molar dentition, the Haplodont, Ptychodont, Bunodont, and 

 Lophodont, are by no means sharply defined, but pass into each other by insensible 

 gradations at many points. With regard to the two types last named, the transi- 



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