GEOGRAPHICAL DISTRIBUTION AND DESCENT. 



213 



to that group. Very probably other ancestors 

 of the opossums will yet be found in America, 

 still it remains a noteworthy fact that the 

 family is restricted to America, and is not 

 represented in Europe subsequently to the 

 Middle Miocene. 



But we must not lay too great stress on 

 the resemblance between the Tertiary Euro- 

 pean marsupials and those of America. Re- 

 cent investigations of Lemoine have shown 

 that the genus Plagiaulax still lived in Tertiary 

 times in the neighbourhood of Reims. This 

 genus has incontestable affinities to the Aus- 

 tralian rat-kangaroos of the present day, and 

 we shall return to them again when speaking 

 of the Jurassic marsupials. 



The earliest mammalian remains of any 

 kind are small teeth, which are found in the 

 transitional breccias of the New Red Marl 

 (Keuper) belonging to the Triassic strata of 

 Degerloch near Stuttgart, and in a bone 

 breccia near Frome in England manifestly be- 

 longing to the same period. To these remains 

 the name of Microlestes has been given. 

 These isolated small teeth resemble most 

 closely those of the genus Myrmecobius among 

 still living forms, and those of Plagiaulax 

 among extinct genera, and on account of 

 this resemblance these remains have been 

 regarded as belonging to a small insectiv- 

 orous marsupial. A lower jaw found in 

 North Carolina and referred to a marsupial 

 called Dromatherium sylvestre is much more 

 complete ; it shows seven three-cusped molars, 

 three sharp-pointed premolars, a slender 

 canine, and three incisors placed at some little 

 distance from each other. 



These teeth thus seem to indicate an insec- 

 tivorous marsupial resembling the still sur- 

 viving genus Myrmecobius and the fossil 

 Spalacotherium of Purbeck. 



We thus see that without doubt it is the 

 insectivorous type of dentition that is most 

 clearly developed in the oldest mammalian 

 teeth. We may well lay special stress on 

 this point, which seems to us to be of peculiar 



importance, for with regard to the form of the 

 crown with several sharp peaks, and the dis- 

 tinction between the different sorts of teeth, 

 molars, premolars, canines, and incisors, the 

 insectivorous is not inferior in richness to any 

 other. The dentition in all the other orders 

 of mammals appears rather to be a modifi- 

 cation or degradation of some kind or other 

 of the original insectivorous dentition, and at 

 the most we can point to the more complex 

 internal structure of certain dentitions as in- 

 stances of a higher development. The con- 

 siderable number of the teeth would rather 

 appear to connect these primitive dentitions 

 of the old marsupials with those of the reptiles. 

 If we assume that the Dromatherium had the 

 same number of teeth in both jaws, then the 

 total number of teeth belonging to it would 

 amount to fifty-six, and even this number is 

 exceeded by that found in the Thylacotherium 

 (Amphitherium) of the Jurassic period, in 

 which the like assumption would give a total 

 of sixty-four teeth. 



The Stonesfield Slates, belonging to the 

 Lower Oolitic strata of the Jurassic period, 

 have yielded many highly interesting lower 

 jaws, some of which (Amphitherium, Amphi- 

 lestes, Phascolotherium) still show the insec- 

 tivorous type of dentition along with the charac- 

 teristic inflexed angle of the lower jaw of the 

 marsupials, while the genus Stereognathus 

 again presents a quite different character. 

 The fragment of the latter that has been pre- 

 served has three molars with crowns which 

 seem to indicate rather a herbivorous or an 

 omnivorous animal. According to Owen these 

 teeth differ considerably from those of all other 

 living or extinct mammals known to us. 

 The form most closely resembling them is 

 found in the second lower molar of Pliolophus 

 vulpiceps, a small hoofed animal of the London 

 Clay(Lower Eocene), which again is connected 

 through the genus Hyracotherium, belonging 

 to the same strata, with the Palseotherida of 

 the Upper Eocene. Other naturalists profess 

 to discover a certain resemblance between 



