348 MESSRS. HANCOCK AND HOWSE 



found lying in regular order, as if forming a portion of a vertical 

 row. 



This curious pile of teeth forms a close, dense mass, increasing 

 in size upwards, or as the last-developed teeth are approached — 

 the smaller rows of teeth, as already stated, being below ; and 

 the teeth themselves are, as it were, interlocked. The central 

 teeth of each horizontal row are the only ones that are placed 

 exactly above each other ; the lateral teeth of the successive 

 rows are arranged in quincunx, so that they may be looked upon 

 as forming slightly diverging diagonal lines, having the central 

 teeth as their starting-point. Now, the first primary lateral 

 teeth, or those next the centre, underlie to some extent the un- 

 der surface of 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 petalodontoid teeth. Thus the whole 

 mass becomes interlocked like a piece of masonry ; or, if we take 

 all the central teeth to form a vertical 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 in- 

 sinuated 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 way affected by the three teeth that conduce to 

 the support of each superincumbent tooth. That this is the fact 

 is apparently demonstrated by the central area being occasionally 

 grooved transversely, corresponding as the grooves do to the 

 imbricated ridges of the crushing-disk of the supporting teeth 

 (PI. X., figs. 1 & 5). 



As a further proof that such is the fact, it may be observed 

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

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

 ponding superincumbent tooth is likewise smooth. It is only 

 when -the ridges are retained that these impressions are observed 

 in the upper teeth ; and indeed were no other evidence at hand, 

 it is patent enough that these peculiar facets are in part the 



