TYPES OF MOLAR TEETH OF MAMMALIA EDUCABILIA. 73 



ized forms ; thus in Homo, Mus, Mastodon, and such genera, the molars of both 

 jaws are identical; in Palceotherium, Eohasileus, Tapirus, etc., and most Camivora, 

 the superior are the wider, the inferior narrower appropriately to the greater 

 slenderness of the mandibular bone. The latter, or anisognathous type, may then 

 be regarded as the more specialized. The Bunodonts, except some Carnivora, are 

 all of the former or isognathous type ; among Lophodonts the few Eodents, the 

 Dinotheriidce, and Elephas are isognathous, while all of the other Proboscidia, the 

 Perissodactyla, and Ruminantia are anisognathous. Examples may be selected as 

 follows : — 



Isognathous; Bunodonts, Homo, Dicotyles, Sus Mastodon, Elotheriwn ; Lopho- 

 donts. DinotheriidcB, Elephas ; Rodentia, Sciuridcs. 



Anisognathous; Bunodonts; Macacus, Lemuroidea, Procyon ; Lophodonts; A710- 

 plotherium, Hyopotamus, Oreodon, Ruminantia, Perissodactyla, Bathmodon, Eohasileus. 



II. The subordinate types of Lophodonts. 

 L The Maxillary Teeth. 



In the essay already quoted* the following remarks (p. 7) explain the relation 

 between the Bunodont genera and several of the Lophodont types of superior molar 

 teeth. " In the superior molar series the flattening of the outer tubercles may 

 proceed so far as to produce, on wearing, a confluence of the [resulting] crescentoid 



surfaces In both Palceosyops and Hyrachi/us these tubercles of the upper 



molars are confluent into two Vs (more or less open when unworn). In the former 

 the inner tubercles retain their primitive conic tubercular form, but in Palceothe- 

 rium, Rhinocerus, Lophiodon, Hyrachyus, and Tapirus they elongate transversely so 

 as to meet the corresponding outer tubercles (now crests) forming the familiar cross- 

 crests of those genera. If alternate, the oblique crests of Palceotherium; if opposite, 

 the cross-crests of Tapirus. If, on the other hand, the inner tubercles flatten like the 

 outer, we have, on wearing, the quadricrescentoid [Selenodont] type of the Rumi- 

 nantia and Anoplotherium. But it is important to observe that the lower types of 

 Quadrumana and Carnivora present the quadrituberculate crown with tendency to 

 flattening of the outer tubercles and the entire loss of the inner, the ' heel' being 

 in the dogs and cats, e.y., their only representative. In the Quadrumanous families, 

 including man, the primitive quadrituberculate type of molars is preserved." 



Four types of Lophodont dentition are included in the above discussion, and 

 three others may be added. They belong to two series, viz., those in which the 



* Primitive Types of Mammalia Educabilia, May, 1873, and Hayden's Report on Geological Survey of Mon- 

 tana, Wyoming, etc., 1873, p. 646. 



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