TERTIARY MAMMALS 115 
only parallel among existing forms to the excessive number of 
molar teeth found in these Mesozoic genera occurs in the Mar- 
supial genus Jfyrmecobius, of which a description is given in a 
succeeding chapter. Jaws more or less closely resembling those 
described under the names mentioned above are also found in 
the uppermost Cretaceous of the United States. A feature com- 
mon to these Mesozoic mammals and J/yrmecobius and some other 
existing forms is the presence of a narrow channel on the inner 
side of the mandibular ramus known as the mylohyoid groove 
(Fig. 29). 
The last type of molar dentition occurring among the Mesozoic 
Mammalia is that found in the 
lower jaws (Fig. 31), upon which 
the genus Spalacotherium was 
established, the upper jaws, 
described as Peralestes, being 
Fic, 31.—Part of the left ramus of the man 
apparently referable to the same dible, viewed from the outer side, of Spala- 
animal. Upper and lower teeth cotherium tricuspidens; from the Purbeck of 
: . Swanage, Twice natural size. (After Owen. 
of this form are represented in Giramnge. Thane namineL Sze. CREM OWE) 
Fig. 4 (6, 7), p. 31, where they are described as typical examples 
of the tritubercular type of molars, the upper teeth having one 
inner and two outer cusps, and the reverse condition obtaining in 
the lower ones. This type of molar presents a marked resemblance 
to that found in the existing Insectivorous genus Chrysochloris ; the 
number of lower teeth in Spalacotherium is, however, 1 3, ¢ 1, 
p+m 10, by which it is widely distinguished from all the Insect- 
ivora. j/enacodon, of the Upper Jurassic of the United States, 
appears to be allied to Spalacotherium. 
Tertiary Mammals.—The more important types of Tertiary 
mammals will, as already mentioned, be noticed under the heads 
of the groups to which they are severally allied; but a few general 
remarks on this subject may be advantageously recorded in this chap- 
ter. In the first place, it may be observed that the comparatively 
scanty evidence of mammalian life hitherto yielded by the Cretaceous, 
coupled with the number and variety of forms approximating to 
the existing groups found even in the lowest Tertiary, indicates a 
great imperfection of the geological record. At present, indeed, 
we have no decisive evidence of the existence of any members of 
the Eutherian subclass previously to the Tertiary; but it can hardly 
be doubted that in some part of the world they had made their 
appearance before that epoch. The Eutherian mammals of the 
lowest Eocene, both in Europe and the United States, are of an 
extremely generalised type; and although many of them approximate 
to existing groups, they show such a combination of characters, now 
restricted to individual groups, as to indicate that several of the 
various orders into which the subclass is now divided were at that 
