OF THE MESOZOIC MAMMALIA. 
231 
internal none with a small posterior heel {Diplocynodon)} Lower molars with hvo 
lofty external cusps and a posterior heel, connected with a. broad crenate internal 
cingrdum. Opposition of upper and loroer molars not trenchant. Premolars with dis- 
tinct ''cingulum’’ and basal” cusps. Canine bifanged. Incisors erect. Condyle 
low. rounded and tipon molar level. Coronoid process elevated, but not very broad. 
Angle short, anteriorly placed, not inflected, always distinct from lower border and 
from condyle. The molars have two fangs placed in line?' 
Although this family belongs in this sub-group as now defined, its members . 
present a more primitive dentition than the following family. The molar pattern is 
a good example of the “ transitional ” type, in which the internal cingulum is giving 
rise to a complete row of internal cusps and to the “ opposition ” pattern. The 
writer has found it very difficult to assign Amphitherium, from the English lower 
Jurassic, its proper position. Its wide distinction from Amphilestes, with which it 
was at first placed by Professor Owen, is shown by the absence of the third cusp ; it 
is thus upon an entirely different line of descent. As observed by Professor Marsh, the 
molar pattern of Diplocynodon, and the allied Jurassic genera, distantly approaches that 
of Amphitherium ; in both genera the molar is bicuspid with a posterior heel which 
extends upon the postero-internal face of the croAvn into a broad crenate cingulum. 
The Amphitherium molar then differs from the Diplocynodon molar in the greater 
development of the anterior cusp — a difference of degree only. The mandibular 
characters of these genera are very similar. This anterior or second cusp is wanting 
in the molars of Peralestes ; the Amphitheriidai may thus be sharply distinguished 
from the Triconodontidce, and less widely from the Peralestidoe as above. 
AMPHITHERIUM, De Blainville,’’ 1838. 
Dentition — 1 i ;, c j, pm j, m g. This formula is somewhat uncertain, being derived 
from a study of Prevost’s and Owen’s drawings, and from the description given by the 
latter. The premolars have posterior “ basal ” and anterior “ cingulum ” cusps. The 
molars have two main cusps and a posterior heel, and an internal cingulum bearing a 
prominent cingulum cusp. 
DIPLOCYNODON, Marsh,* * 1880. 
Dentition — ii, c J, pm j, mi. The jaw is elongate and gently curved below. 
The mylohyoid groove is parallel with the lower border of the ramus. The coronoid 
is large and elevated. The condyle is nearly upon a line with the teeth. The 
* Marsh “ Am. Jur. Mamm.,” p. 338, 1887. The writer has not examined the upper molars of this genus. 
2 This family is equivalent to the Diplocynodonticlx, Marsh, 1887. 
« Comptes Rendus, Aug. 20th. “ Doutes sur le prdtendu Didelphe fossile de Stonesfield.” Unfortunately 
I have been unable to procure good figures of the internal aspect of the teeth of this genus. 
* “ Am. Jour. Sc. and Arts,” 1880, p. 235. Diplocynodon is preoccupied by Pomel. (See Appendix). 
