98 JURASSIC MAMMALS CHAP. 
the family, is Plagiaulaz. As it is not Triassic, the consideration 
of its characters will be deferred until later. MJicrolestes is a 
Rhaetic genus, known from rocks in Germany and England ; 
but it is entirely based upon molar teeth. JL antiquus has a two- 
rooted molar of an elongated form with a row of tubercles on either 
side of a median groove, which traverses the long axis of the 
tooth. To some extent the teeth of the ancient form resemble 
those of Ornithorhynchus.  Microlestes has been sometimes spoken 
of as a Marsupial, but Mr. Tomes! has found that it does not 
show one very universal character of the Marsupial teeth : 
it has not those continuations of the dentinal tubes which 
traverse the enamel in all Marsupials that have been examined 
with the sole exception of the Wombat. 
The rarity of the remains of mammals in these earliest rocks 
of the Secondary epoch has been accounted for in another way 
from that which has been suggested above. It may be that the 
croup Mammalia was not evolved in Europe at all, and that the 
stray remains which have been found in that continent represent 
the fragmentary remnants of a few scattered immigrants which 
heralded the later invasion of more numerous genera during the 
Jurassic period. 
The Mammals of the Jurassic Period.—Some of the Allo- 
theria or Multituberculata described in the last section occur in 
the rocks of this early part of the Secondary epoch. They are 
doubtful in position, as already stated; some of them indeed, 
as for instance 7ritylodon and Dromatheriwm, are possibly not 
mammals at all, while the remainder probably belong to a non- 
existent order of mammals. Along with these dubious creatures 
are the fragmentary remains of small animals which are not 
merely mammals, but in all probability definitely Marsupials. 
It is true that here again we have little beyond lower jaws 
and teeth to deal with; so that there may be less certainty 
in referring them to the Marsupials than appears to be the 
opinion of the majority of Palaeontologists. 
Professor Osborn in fact considers that the Mesozoic mammals 
consist of three groups: (1) The Multituberculata, including 
the Bolodontidae, Stereognathidae, Plagiaulacidae, Polymasto- 
dontidae, and possibly the Tritylodontidae (which, however, are 
regarded by him and by others as more probably reptiles of the 
1 Dental Anatomy, 5th ed. 1898, p. 304. 
