AMPHITHEKITD-^. 269 



Subfamily Myrmecobiin^. 



(^Although unrepresented in a fossil state, it is important that this 

 group should he mentioned here.^ 



Dentition: — Pm. + M. q, and of small size; the three premolars are 

 not well differentiated from the true molars ; the lower incisors are 

 spaced; the true molars are subquadr angular and multicuspidate, 

 the lower ones not being differentiated into a blade and talon ; the 

 mandibular ramus is shallow, with a mylohyoid groove. There is 

 no evidence of the existence of a milk-molar preceding the last 

 premolar. 



Family AMPHITHERIIDiE \ 



The genera provisionally included in this family comprise a 

 number of small primitive mammals, apparently connected on the 

 one hand with the Myrmecobiine branch of the Dasyuridce, and on 

 the other with the Didelphyidce. It may be eventually necessary 

 to divide the family ; and it is' difficult with the present imperfect 

 evidence to give a distinctive diagnosis which will embrace all the 

 forms. They all, however, have a mylohyoid groove in the 

 mandible ; their lower true molars, which (with the possible excep- 

 tion of Phascolotherium) exceed four, are either multicuspidate, 

 tricuspidate, or differentiated into a blade and talon ; the premolars 

 may be reduced to three or perhaps two, but are frequently four, 

 and have been thought in some instances to exceed that number ; 

 the lower incisors are either three or four ; and the canines have 

 grooved or double roots. It is not known whether the last pre- 

 molar had a milk predecessor. 



The difference in the characters of the lower molars in the various 

 genera is not greater than obtaining in the Dasyuridce. If the 

 number of premolars in some members of the first section exceed 

 four, it is a unique feature among heterodont mammals. 



Section A. — The lower true molars multicuspidate, or tricuspidate 



and cingulated ; the lower incisors spaced, and either three or 



four in number ; and the premolars in some instances reduced 



below the latter number. 



The American Triassic Dromatherium ^ may not improbably be 



included in this section: the dental formula being I.|, C. ^, Pm..- 



' Owen, British Fossil Mammals and Birds, p. 29 (1846). 

 2 See Osborn, Proc. Amer. Phil. See. vol. xxiv. p. 109 (1887). 



