OF THE MESOZOIC MAMMALIA.- 
195 
between the coronoid border and the canine, suggests rather that premolars were pres- 
ent in members of the ancestral line of Phascolotherium, and have disappeared in 
this genus, the. true molars only having been retained. The diminutive size 
of mi and suggests that a still further reduction of the series is in progress at each 
end. The molar pattern consists of a prominent central cone with smaller anterior 
and posterior cusps sub-equal in size, upon its slopes. There is a very pronounced 
internal cingulum encircling the crown and rising into anterior, posterior and median 
basal cusps as in ArnpMlestes} Faint vertical ridges may be observed upon the main 
cusp. According to the writer’s observations the lateral cusps project partly from the 
inner slopes of the main cusp, and are not precisely in line. 
TRICONODON, Plate VIII, fig. 4. 
This genus, from the Purbeck, is represented by numerous remains of both jaws, 
and is therefore the best known of all the Mesozoic Mammalia. It is distinguished 
by many “ recent ” characters, and forms an important link between three of the 
Stonesfield Slate genera and some of the modern Marsupials. 
As shown by Professor Owen, the mandibular rami vary in different species. In 
T. ferox, which has been selected as the type in this memoir, the lower border has a 
single downward curvature, slightly raised beneath In T. mordax the angle of 
the jaw is slightly depressed below the condyle, but in T. ferox it is sharply and 
widely inflected so as to partially obscure the condyle, recalling the Phascolotherium jaw. 
The condyle is usually upon the molar level, or rather the level of the alveolar bor- 
der; in some specimens, as in the T. mordax (Mes. Mamm., PL III, fig. 7), the strongly 
convex articular face of the condyle is directed backward ; in another specimen, pro- 
bably T. ferox, the condylar face was seen directed upwards (compare Mes. Mamm. 
PI. Ill, fig. 2, B). The supracondylar notch in T. ferox is much deeper than in T. mor- 
dax. The coronoid process is in all cases broad and powerful with rather shallow fossae 
and a swollen anterior border. In the lower angle, on the inner face is the dental fora- 
men, and from this the shallow mylohyoid groove extends forward near the lower 
border. The inner face of the ramus is vertically convex, and indicates great strength; 
the outer face is flat below the alveoli and bulges near the lower border. In some speci- 
mens (Mes. Mamm., PL III, fig. 7,) a shallow groove extends forward from the crota- 
phyte fossa ; below this fossa is a ridge extending backwards to the outer side of the 
condyle. Below in T. mordax, the lower border bends sharply upwards to form 
the stout chin ; this character is unique among the mammals of the period. 
Mandibular dental senes in T. ferox. The incisors as preserved in this species 
and in T. occisor are represented by 4 and 4 ; there are also traces of the alveolus of 4. 
* The median basal cusps are slightly less prominent than in Amphilestes. They are omitted in the text 
and drawings given by Professor Owen, who describes the molars as “quinquecuspid,” but may be seen in the 
figures given by Buckland, Bridgewater Treatises, Geology and Mineralogy, Vol. II, Plate 2. 
