MESOZOIC MAMMALS 113 



Jurassic of North America ; and the Plagiaidacidce 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 (Meniscoessus 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 

 (Triacanthodori), in which the formula of the lower teeth is i 3, c 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 an tero- posterior line, those of the 



i i . , . i ., n . Fio. 28. Reversed view of the 



upper and lower jaw being alike. Pna- left rainus of ^ mandible of 



COdon, of the Jurassic Of the United States, Triconodmt, mordax ; from the 



is probably inseparable from Tricmod&n. Purbeck of 8wana g'. Natural 



T \, * r>7 j j? /T-L- o\ f Slze - (After Owen.) 



In the genus Phascolot/ienum (iig. 29) of 



the Lower Jurassic Stonesfield Slate, the lower teeth may be 

 classified as i 4, c 1, p 3, m 4, the premolars and molars being 



8 



