On the Mesozoic Mammalia. 2F 
work now before us, these are brought up to no less than thirty- 
five, five of which come trom the Trias. A very convenient 
table shows theorder of occurrence of these forms in time, so far 
as they are now known to us, the arrangement in the columns 
presenting the beds of England, North America, Africa, Austra- 
lia, and the Continent. It is pointed out by our author, that, 
“In arapid survey of this ancient fauna, we are at first struck with 
the very great diversity which prevails in the form and arrange- 
ment of the teeth, consisting of six or seven wholly distinct types; 
and this at a zoological period which we have been accustomed 
to consider as the dawn of mammalian life. The above types, 
although primitive, are essentially mammalian. In one genus 
only, Dromatherium, do we find clear evidence of reptilian affin- 
ity in the dentition. Then we are surprised to discover a very 
close zoological relationship between fossil faunze of the same 
age, but having a wide geographical distribution. The most 
striking instance of this is the parallelism between the American 
and British upper jurassic fauna,” These mesozoic mammals 
were characterized by their diminutive size, and the excessive 
number of teeth they possessed, which latter, otherwise, are less 
~ archaic in their pattern than we would be led to believe from a 
mere casual observation; the molars being in unusual numbers. 
As his task developed, Professor Osborn was led to lay especial 
stress upon (1) the generic characters of the British mesozoic 
forms; to show (2) their relationships to modern orders, that is 
all the known mesozoic species to the existing orders of mammals; 
and (3), a full discussion of the dentition in all its bearings. This 
is the fundamental plan of the monograph before us, and upon 
these lines it has been worked out with marked ability and clear- 
ness. The literature of the subject has been carefully gone over, 
and previous laborers in the same fields given in every instance, 
full credit for their productions. Space will not admit of our 
entering in detail upon the admirable classification adopted in 
this monograph, be it enough to say that it is based almost ex- 
clusively on the dentition, and that, ‘‘We first observe that the 
Mesozoic Mammalia divide into two large groups. In the first 
group, A, one of the incisors is greatly developed at the expense 
of the others, and of the canine, which usually disappears; be- 
hind these teeth is a diastema of varying width, while the molars 
bear numerous tubercles. In the second group, B, the incisors 
are small and numerous, the canine is always present, and well 
developed; the teeth usually form a continuous series, and the 
molars bear cusps instead of tubercles. These two divisions sug- 
gest those which obtain among the modern Marsupials, but are 
in fact much more sharply defined and widely separated from 
each other.”’ 
In conclusion, we find a general discussion upon the develop- 
ment of the teeth of these mammals in time, and masterly sec- 
tions devoted to the zoological position of these mesozoic types 
in the two groups just mentioned, to their relations to the Mar- 
