ON TIIK STRUCTITKP] AND CLASSIPNCATION 
• 22 « 
molars arc of an iiitermoiiiate form. Arnphitylus, from tlie same strata, stands near it 
(althougli not very fully known and presenting some features exceptional in this sub- 
group); while Triconodon, from the Upper Jurassic, is much modernized. Phascolo- 
therium, of the second sub-family, from the Lower .Jurassic, apparently lacks preraolari- 
form teeth, but is linked with the other Triconodontidoi by Tinodon (Upper Jurassic) 
which has premolars. The molars in each of the latter genera are intermediate 
between those of Amphilestes and Spalacotherium, which, in turn, is the type of the 
third sub-family. The primitive genera have four incisors, but the lateral incisors, 
disappear in their successors. 
a. AMPHU.ESTISM} 
.Molar cusps in same fore-and-aft line; little or no reduction of the premolar 
series ; angle distinct. 
AMPHILESTES, Owen,^ 1871. 
Dentition — i^, c., pnij, m,. The posterior basal cusps of the premolars, if 
pre.sent, are not fully distinct from the cingulum. The anterior basal cusps belong to 
the cingulum. The middle cusps of the molars are much more prominent than the 
anterior and posterior. The molars are separated, the internal cingulum projecting 
beyond the anterior and posterior slopes of the crown. The condyle is distinct from 
the angle and on the molar level. 
AMPHITYLUS, Osborn,’ 1887. 
Dentition — i,^^ c^, pm^, m^. The premolars ^ 
resemble those in the last genus. The molars 
have three blunt, compressed cusps, and are 
compactly placed. The cingulum is less promi- 
nent and does not embrace the anterior and po.s- 
terior slopes of the crown. The condyle is lofty 
and pedunculate. 
TRICONODON, Owen,'* I860. 
Syn. IVku-anthodon, Owen,® 1871. 
Dentition— 1 1 c 1, pm 4, m i or 4. As shown upon page 196 of this memoir, the 
fourth molar comes into place only in old individuals or is developed as a specific 
chamcU-r. In young individuals the fourth premolar, d,, is molariform and is shed 
early, riie premolars have prominent posterior and sometimes anterior basal cusps. 
rho molars have three sub -equal trenchant cusps. The canines are stout and re- 
' Sub. fain. nov. 
rr 
Fiouhk 10b. Amphitylu* Oweni, after Owen 
(.1. 1‘rfrotiii), two and one eighth times 
nalnntl eiie, from the Bnckland collection. 
’"Meeoioic Mammalia,” p. 16. 
Encyclop. Bnt.. Vol. 17, 1859. Art. Paleontology (fide Owen). 
Mceotoic Mammalia/' p. 72. 
