OF THE MESOZOIC MAMMALIA. 
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theriidce and Peralestidce depends entirely upon the homologies of the cusps. The 
position of the Amhlotheriidce is also uncertain, since their molar structure is not fully 
**HypotheticaL 
known. The Dromotheriidae, the only representatives of the Protodonta, are consid- 
ered somewhat aberrant because of the wide diastema behind the canine and 
the presence of but three premolars. 
A. — First Group. 
We have at present but little insight into the derivation of the multi tuberculate 
dentition. In the oldest known genera the dental series has already undergone con- 
siderable reduction and a much higher degree of specialization than is attained by 
any of the mammals of the recent group. The most prominent features of the denti- 
tion are the hypertrophy of a pair of incisors in each jaw, the atrophy of the remain- 
ing incisors and the canines, the reduction of the premolar series, the longitudinal 
rows of tubercles upon the molars and the wide diastemata. 
Incisors. — Among the genera in which the mode of reduction has left any record 
we find the second incisor, or rather one of the lateral incisors hypertrophied. In the 
Bolodontidm, as demonstrated by Marsh in his observations upon Allodon, the median 
incisor is atrophied and the second incisor hypertrophied. In Bolodon the median 
incisor is apparently suppressed, and the third is much smaller than the second. Of 
the two incisors in Tritylodcm, the outermost is close to the maxillary suture, the 
hypertrophied incisor is close in front of this and widely separate from its opposite 
