348 THE VERTEBRATE SKELETON. 



MAMMALIA 1 . 



It will be well here to briefly refer to certain mammals 

 of small size, the remains of which have been found in de- 

 posits of Mesozoic age. In the great majority of cases they 

 are known only by the lower jaw, or sometimes only by isolated 

 teeth. A large number of them are commonly grouped 

 together as the Multituberculata, and are sometimes, partly 

 owing to the resemblance of their teeth to those of Ornitho- 

 rhynchus, placed with the Prototheria, sometimes between the 

 Prototheria and the Metatheria. They are characterised by 

 having a single pair of large incisors in the lower jaw, and one 

 large with one or two smaller incisors in each . premaxilla. 

 The lower canines are very small or altogether wanting. The 

 incisors are separated by a diastema from the grinding teeth, 

 which are sometimes (Tritylodon) characterised by the posses- 

 sion of longitudinal rows of little tubercles separated by 

 grooves, sometimes by having the premolars provided with 

 high cutting edges, whose surfaces are obliquely grooved. 

 Some of the Mesozoic mammals found associated with the 

 Multituberculata, have however a dentition of an altogether 

 different type, with at least three lower incisors, well developed 

 canines and premolars, and numerous molars with peculiar 

 three-cusped or tritubercular grinding surfaces. These mam- 

 mals, one of the best known of which is Phascolotherium, are 

 commonly separated from the Multituberculata, and are divided 

 by Osborn into two groups, one allied to the Marsupials, and 

 one to the Insectivores. The group showing Marsupial affinities 

 is further subdivided into carnivorous, omnivorous, and herbi- 

 vorous subgroups. The members of both groups commonly 



1 See K. Owen, "Monograph of the Fossil Mammalia of the Mesozoic 

 Formation," Pal. Soc. Hon. 1871. 



H. F. Osborn, "Structure and Affinities of Mesozoic Mammals," 

 J. ofPhilad. Acad. 1888, vol. ix. 



O. C. Marsh, "Jurassic Mammals," Amer. J. Sci. 1878 et seq. 



