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 Mouo- 

 tremes the AUotheria 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 ISTorth 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 Eodent-like in form, 

 and not very different from those of certain Marsupials. This 

 point of difference cannot be regarded as of very first-rate im- 

 portance ; no one would relegate the Sloth and the Armadillo to 

 different orders on account of their tooth diiferences, 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 AUotheria 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 



