fruiii tlic Marl- Slate of Mldderidytf. 55 



upon as l'ormin<^ slightly diverging diagonal lines, having the 

 central teeth as their .starting-j)oiiit. Now, the first ])riniary 

 lateral teeth, or those next the centre, underlie to some ex- 

 tent the under surface (tt" the central teeth ; and the second 

 primary lateral underlie in a similar manner the margins 

 of the first primary, and so with the third or pctalodontoid 

 teeth. Thus the whole mass becomes interlocked like a piece 

 of masonry ; or, if we take all the central teeth to form a ver- 

 tical row, and consider in like manner the various lateral 

 teeth, then it might be said that the teeth composing such 

 vertical rows had their lateral margins insinuated between 

 those of the adjacent rows. 



In consequence of this interlocking and close approximation, 

 the back or under surface of each tooth becomes worn, and 

 the three longitudinal areas or facets, already described, become 

 more strongly defined. The central area and the two lateral 

 areas are in this Avay affected by the three teeth that conduce 

 to the support of each superincundjent tooth. That this is the 

 fact is apparently demonstrated 1)y the central area being occa- 

 sionally grooved transversely, coiTesponding as the grooves 

 do to the imbricated ridges of the crushing-disk of the sup- 

 porting teeth (PI. II. figs. 1 & 5). 



As a further proof that such is the fact, it may be obsei-ved 

 that when the crushing-disk has by previous use been worn 

 smooth, which frequently occurs, the central facet of the cor- 

 responding sujMjrincumbent tooth is likewise smooth. It is 

 only when the ridges are retained that these impressions, aje 

 observed in the upper teeth ; and, indeed, were no other evi- 

 dence at hand, it is patent enough that these peculiar facets 

 are in part the result of wear ; for they exhibit on their sur- 

 faces the internal structure of the matter composing the tooth. 

 And that the opposing crushiivg-disk is not equally and mutu- 

 ally worn arises from the fact that it is covered Avith a layer of 

 hard enamel-like matter. 



The existence of the transverse grooves would seem also to 

 prove that while they were produced by the rubbing-motion 

 of the teeth upon each other, the motion itself must have been 

 very limited, or neither the grooves nor the sharp (lefinition 

 of the facets could have existed. And in this way we have a 

 corroboration that the retention of the old, effete teeth is 

 merely for the mechanical sujjport that they supply to the 

 upper row of teeth, ujjon which teeth alone devolves the func- 

 tion of cutting and crushing the food. 



The four groups of teetli obtained by Mr. Duff at Midde- 

 ridge are very instructive, an«l, though in a more or less dis- 

 turbed state, are quite sulHcient to show the original disposition 



