Review of Recent Geological Literature. 65 
arc largely duo to the labors of American palaeontologists working on 
the abundant material that the new states of the west have afforded. 
The chasm existing between the mammals and the reptiles is now 
more obvious than that which formerly existed between the latter and 
the birds. But just as the one has been almost filled up, so the other is 
rapidly becoming less profound and abrupt. The few links supplied by 
the scanty existing marsupials and monotremes are of inestimable value, 
while the fact that all the most ancient fossil mammals belong to one 
or other of these groups is intensely significant as indicating the line of 
descent. 
Only three monotremes are known now to be in existence, the duck- 
bill and two ant-eaters in Australia and in New Guinea, and these the 
author says, cannot be regarded as ancestral types. It is worthy of re- 
mark that though the monotremes hold the lowest rank among theMam- 
malia, yet the oldest mammalian fossils are placed by all among the mar- 
supials — an inversion of the order of development probably due to the 
imperfection of the record. This is far from surprising when we look 
at the very scanty nature of the material that we possess regarding these 
<'arly animals and that this little is in most cases limited to a few lower 
jaws and teeth. No fossil monotreme has yet been found of earlier 
date than the Upper Triassic and these are only two detached teeth of 
Microlestcs and one of Triglyphus (Tritylodori) . The slowness with 
which the early mammalian remains are coming to light is very remark- 
able. Nearly fifty years have passed since Prof. Pleininger found the 
two teeth and a few small fragments of bone of Microlestcs near Stutt- 
gard, and Prof. Emmons met with three jaws of Dromatlicvlum sylvestre 
in North Carolina. Yet in all this long interval no other specimen of 
these has been discovered and only one species has been added to the 
scanty list in a single tooth of Tritylodon. Nature seems to be unwill- 
ing to disclose her first imperfect attempts at producing the Mammalia 
— she hides "her 'prentice han'. " 
From the Triassic to the Eocene, over-leaping the Jurassic and Cre- 
taceous systems, we must pass before we find the earth tenanted by any 
higher form of mammalian life than the marsppials. In the Eocene we 
meet the earliest specimens of the Edentata, or sloths, a peculiar order 
widely differing from these above them in their imperfect dentition 
which exhibits some indications that may be const rued to imply degen- 
eration. 
It is not a little singular that no remains of fossil sloths have 
been found in Europe, nor does any animal of this order inhabit that 
continent at the present day. South America is now. and apparently 
has always been the metropolis of slothdom. There lived the Qlypto- 
dons, huge armadillos six and eight feet in length; Megatherium, SceU- 
dotherium and Mylodon, sloths rivalling the Rhinoceros in bulk and 
capable of pulling down for food, the trees, which they were unable to 
climb. 
Most of the fossil Edentata are of late Tertiary date Pliocene or even 
