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 {98} the family, is _Plagiaulax_. As it is not Triassic, the consideration of its characters will be deferred until later. _Microlestes_ is a Rhaetic genus, known from rocks in Germany and England; but it is entirely based upon molar teeth. _M. 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[48] 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.