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 Monotremes 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 importance; 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
