220 bulletin: museum of comparative zoology. 



against the mandibular in the manner already described. On bringing 

 the two surfaces together, all marks of wear observable in the anterior 

 palatal plate are seen to coincide with similar worn areas of the opposing 

 plate, thus fully disclosing their mutual relations. 



The ental margin of the mandibular plate fits just within and against 

 the side of the ental ridge of the palatal plate, and closes against the 

 thickest and highest portion of the ridge near the antero-internal angle, 

 where both plates are deeply worn. The ectal rim of the mandible fits 

 accurately into the broad and deeply worn concavity of the palatal 

 plate, the surface of the one being a faithful replica of the other, and 

 the relations between them being comparable to those of a die-stamp. 

 The extremely prominent bifid eminence which rises midway the length 

 of the mandibular plate along its ental margin functions within the 

 depressed posterior surface of the palatal, and the longitudinal cleft by 

 which the eminence is divided embraces the elongate, mesially placed, 

 and longitudinally directed tubercle of the palatal plate, whose summit 

 fits into a shallow groove of the mandibular plate. Thus an inter- 

 relationship between all the parts is demonstrable, which must faith- 

 fully indicate their natural arrangement, since no other is capable of 

 producing the observed effects. Having ascertained the relations of the 

 anterior palatal plates, it is an easy task to bring the hinder one into 

 adjustment with it and with the remaining portion of the lower 

 dentition. 



The posterior palatal plate may be described as approximately cordi- 

 form, or subtriangular. Not more than two of its borders are straight 

 or slightly concave, the third being profoundly indented, and for that 

 reason incapable of direct contact with the preceding element. The 

 contact margin must therefore have been formed by one or the other of 

 the nearly linear sides, and by experimenting with them in connection 

 with the two plates whose relations have already been determined, it 

 is readily perceived which one of these sides permits of harmonious 

 adjustment. The pointed anterior extremity of the plate we are now 

 considering adapts itself regularly to the outer contour line of the man- 

 dible ; its worn centrally placed tubercle fits into a depression of the 

 lower plate; and its marginally situated tubercle closes just back of the 

 elevated prominence of the mandibular plate, playing into a declivity 

 that occurs on the inner face of the latter. The surface irregularities of 

 both plates, together with all their indications of wear, are thus fully 

 accounted for by this arrangement. Noteworthy also is the fact that 

 precisely similar relations obtain in Dinomylostoma, where the orien- 



