TYPES OF MOLAR TEETH OF MAMMALIA EDUCABILIA. 



81 



as one of "inexact parallelism;" a condition supposed by the writer to depend on 

 modification in descent under the law of acceleration. The language used is:* 

 " The young tooth of Elephas, moreover, is represented by a series of independent 

 parallel laminse at first, which when they unite, form a series of crests similar to 

 the type \i. e. pattern] of the genus Mastodon [Stegodoii'] and others of the beginning 

 of the series. The deposit of cementum takes place later, till the valleys are 

 entirely filled up. Thus the relations of this part of the tooth structure in the 

 series are also those of the successional growth of those of Elephas or the extreme 

 of the series." The history of the origin of this type of dentition is no doubt 

 similar to that of the Selenodonts. 



The transition from the bunodont type to the Fis. le. 



present one in the mandibular dentition is seen in 

 the Eocene genera Microsyops and Liinnotherium, 



where the opposite cones are connected by a low Mwosyops eUgans. 



cross-crest. 



II. Amcebodonts ; Bunodont type Hi/opsodus.f 



Fio;. 17. 



Pis. 18. 



JJi/ojisodu8. 



4. Symhorodonts. — In Hyopsodus the ex- 

 terior cones are already somewhat excavated 

 on the inner side, so that a section" of each 

 is somewhat triangular. It is obvious that 

 but little more compression and curvature 

 is required to produce the type of Palceosyops, etc. (Fig. 18). 

 The angles of the outer cones in Hyopsodus are also slightly 

 produced as low ridges to the bases of the alternating tuber- 

 cles of the opposite side : the elevation of these ridges is only necessary to produce 

 the two Vs of the mandibular dentition of Palceotherium, Palceosyops (Fig. 20), 

 Symborodon, AticJnppodus (Fig. 19), and all their allies (Fig. 21, Palceotherium). 



Palceosyojis hevidens 



Tig. 19. 



Fig. 20. 



Fio;. 21. 



Anchippodue. 



Paheotli 



'lerium.j. 



osyaps. 



* Origin of Genera, p. 17. 



t I originally (On the Primitive Types of Mammalia Educabilia, p. 9; Hayden's Geolog. Survey Terr., 1872, 

 1873, p. 648) selected Oligotomus, Cope, for this position, but as it has a considerable diastema, it is probably 

 better exchanged for Hyopsodus, where the dental series is uninterrupted. 



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