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Bashford Dean Meynonal Volume 



edge where the "tooth" works against the corresponding part of the opposite "tooth." 

 These cutting edges are placed on the inside in the upper jaw elements; in the infero- 

 gnathal, on the outside (Text-figures 27, 28, 29, 30, 31, 32 and 78). If we examine them 

 it is easily seen that they are remarkably plane. When the specimens are not crushed or 

 bent, this plane surface is maintained for the whole length of both IG, ASG and PSG. 



It IS very usual to compare the working 

 of the gnathal elements in Dinichthys with a 

 pair of scissors. This comparison is very 

 adequate. The cutting edges of the jaws are 



Text-figure 77- 

 The position of the gnathal elements PSG and IG 

 in Dinichthys A and C, according to older recon- 

 structions; B and D. m the author's opinion. 

 a-b, movement axis of the head. 



Text-figure 78. 



■ The position of the infero-gnathal 



plates seen from the front, according 



to the author's idea. 



thus like the inner surface of the scissors. The inner surface of each half of a pair of 

 scissors is placed in the same plane, perpendicular to the movement axis of the scissors. 

 When the jaws of Dinichthys work like scissors their cutting edges also must be placed 

 perpendicularly tg the axis of the head movement and parallel to each other (Text-figure 

 77B and D). As we know, the axis of the head movement is strongly horizontal. Thus 

 the cutting edges of the "teeth" have to be strongly vertical. This arrangement of jaw 

 elements in Dinichthys has never before been proposed. Always, in all mounted speci- 

 mens or reconstructions the surfaces of the cutting edges are placed at a smaller or larger 

 angle to the vertical line (Text-figure 77 A and C). But such a position of the jaws ex- 

 cludes the possibility of preserving their plane and sharp cutting edges. As seen in Text- 

 figure 77A, in use their edges are constantly worn and in time they would come to look 

 more like the crushing teeth of Mylostoma. On the other hand, if the cutting edges are 

 placed vertically (Text-figure 77B), they wear evenly along their whole inner surfaces 

 and preserve their sharp edges. 



Usually in reconstructions of Dinichthys, we find the outer surfaces of the gnathal 

 elements in a vertical position (Text-figure 77A and C). As we now know that this was 

 not the case, we can place them more correctly. The new position has only a slight in- 

 fluence on the upper jaw — it becomes somewhat more curved. The position of the lower 



