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 Myrmecobius, 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 Myrmecobius 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 Spctiacotherium was 

 established, the upper jaws, 

 described as Peralestes, being; 



, f ,, i ° Fig. 31.— Part of the left ramus of the man 



apparently referable to the Same clible, viewed from the outer side, of Spcda- 

 ailimal. Upper and lower teeth cotherium tricuspidens ; from the Purbeek of 



of this form are represented in Swanage " Twice natural size - (After Owen.) 

 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, i 3, c 1, 

 p + m 1 0, by which it is widely distinguished from all the Insect- 

 ivora. Menacodon, 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 



