CHAP. IV FOSSIL JAWS 97 
have resisted the decay which would more readily affect the 
softer bones. Where there are bones it is frequently the lower 
jaw alone which has been preserved for us—a bone which has also 
been preserved in the case of some of the contemporary Marsupials. 
It has been pointed out (from the observation of dead dogs 
floating in canals) that the lower jaw is occasionally detached 
from the carcase. It is the most readily separable part which 
contains a skeleton. It may be, therefore, that the remains of 
these early mammals, floating down some river to the sea, may 
have lost their jaws while in the river, or at furthest in the 
shallow waters of the sea, the rest of the carcase floating out to 
a greater distance, and being finally entombed in the stomach 
of some carnivorous fish, or in the mud at the bottom of a deep 
ocean, which has never since seen the light. 
The characters of this group are really more those of the 
Monotremata than of the Marsupials. The undoubted likeness 
which their molar teeth show to the temporary teeth of the 
Platypus have already been commented upon. Like the Mono- 
tremes the Allotheria appear to have possessed a large and 
independent coracoid; the evidence for this rests upon the 
discovery of the lower end of a scapula of Camptomus, a Cretaceous 
genus from North America upon which there is a distinct facet 
for the articulation of what can have been nothing else than a 
coracoid. On the other hand they differ from the Monotremata 
by the presence of incisor teeth which were Rodent-like in form, 
and not very different from those of certain Marsupials. This 
point of difference cannot be regarded as of very first-rate 1m- 
portance ; no one would relegate the Sloth and the Armadillo to 
different orders on account of their tooth differences, which are 
about on a par with those to which we have just referred. It 
seems indeed likely that it will be ultimately necessary to rub 
out the boundary line which now divides the Allotheria and the 
Monotremata. 
The Plagiaulacidae are unquestionably mammals, and they 
are placed by most naturalists in this at present uncertain group 
of Multituberculata, which will be retained here in deference to 
the distinguished authorities who have instituted the group, 
though there are but few characters by which it can be defined. 
This family though appearing in the Trias, extends down in time 
to the Eocene. The type-genus, that which has given its name to 
VOL. X H 
