MESOZOIC MAMMALS 113 
Jurassic of North America; and the Plagiaulucide also persisted 
into the lower part of the Eocene division of the Tertiary period ; 
Neoplagiaulax being a Tertiary form common to Europe and the 
United States, while Liotomus and Ptilodus are at present known 
only from the latter country. 
The present group is also represented in the upper Cretaceous 
of the United States by Selenacodon (Meniscoéssus in part), Cimoliomys, 
etc. Polymastodon, of the Lowest or Puerco Eocene of New Mexico 
is the largest known form, and is characterised by the presence 
of only one premolar and the elongated molars. The angle of 
the mandible is inflected after the Marsupial fashion. 
Polyprotodont Types.—The second type of mammalian dentition 
found in the Mesozoic period resembles that occurring among 
recent Polyprotodont Marsupials—that is to say there are at 
least three lower incisors, the canines are well developed, and the 
premolars and molars are cuspidate, the number of the latter reach- 
ing in some cases to seven or eight. There has been much dis- 
cussion as to the taxonomic position of these forms, and while the 
majority of writers admit the Marsupial affinities of at least a 
moiety, it has been contended that others indicate distinct ordinal 
groups more or less closely allied to the Insectivora. At present, 
however, there is no decisive evidence to support such a view. 
Important proof of the Marsupial affinity of one of these forms is 
afforded by the replacement of the teeth, which appears to be of the 
same nature as in the existing Marsupials, that is to say, the last 
premolar alone is preceded by a milk-tooth. 
The most generalised forms appear to be Dromatherium and 
Microconodon, from Lower Mesozoic beds in the United States, of 
which enlarged views of the teeth are given in Fig. 4 (1, 2), p. 
31. Professor Osborn points out the extremely simple character of 
these teeth, and it is quite possible that these forms may prove 
to be Prototheria. There are three premolars and seven molars in 
the lower jaw of Dromatherium. 
A common form in the Purbeck of Dorsetshire is Triconodon 
(Triacanthodon), in which the formula of the lower teeth is 23, ¢ 1, 
p 4, m 3-4. A lower jaw is shown in 
Fig. 28, and an enlarged view of a molar 
tooth in Fig. 4 (5). The molar teeth con- 
sist of three flattened cones placed in the 
same antero-posterior line, those of the ; 
. . . . Fic. 28.—Reversed view of the 
upper and lower jaw being alike. Pria- left ramus of the mandible of 
codon, of the Jurassic of the United States, Triconodon mordax ; from the 
is probably inseparable from Triconodon. ac ieee Oa Aauatal 
In the genus Phascolotherium (Fig. 29) of ~~ ‘ 
the Lower Jurassic Stonesfield Slate, the lower teeth may be 
classified as 7 4,¢ 1, p 3, m 4, the premolars and molars being 
