A GLANXE AT THE MAMMALIAN DAWX. 



BY B. M. UNDERHILL, V. M. D. 



The identification of the fossil remains of mammals is 

 beset with difficulties, for, while such fossils as shells and 

 skeletons of vertebrate aquatic animals are often found entire, 

 the relics of animals of terrestrial habit are for the most part 

 fragmentar)' and may consist of a portion of bone, or dis- 

 placed groups of bones, teeth or hard products of the skin ; 

 very rarely the complete skeleton. The earliest trace that we 

 have of mammals appears in the Upper Triassic, but these 

 are so few and incomplete, consisting of teeth and portions of 

 jaws, that it is difficult to make out their affinities. Practic- 

 ally all that can be said as to the specimens thus far recov- 

 ered is that they indicate small insectivorous marsupials, 

 ranging from the size of a mouse to that of a rat, to which 

 the opossum is a farriiliar modern animal somewhat allied. 



The hypotheses held as to the relations of these scanty 

 Triassic remains to the Tertian*- mammals have been more or 

 less contradictory. Some would remove them from the mam- 

 mals entirely, claiming that they belong with a group of 

 mammal-like, anomalous, toothed reptiles ; others believing 

 them to be representatives of a mammalian type reaching 

 down to the reptiles, to which they are linked by the egg- 

 laying Monotremes. While in the light of our present know- 

 ledge any conclusion upon the ultimate beginnings of the 

 mammals must be largely hypothetical, there is a concurrence 

 of present opinion that the unknown reptilian stem from 

 which they evolved split into two branches, one of which, 

 the Monotremes, sur\'ives with much of its primitive form in 

 Australia, while from the other branch there arose two divi- 

 sions, one giving rise to the marsupials, which with the 

 Monotremes constitute the Implacental Mammals, the other 

 developing the Placentalia. which includes all of the remain- 

 ing' orders. 



At the beginning of the Tertiary we pass from the myth- 



