AND AFFINITIES OF PTYCHODUS. 



127 



developed teeth of the same series, and there is a corresponding 

 diminution in length. 



Remains of a second lateral row are preserved in contact with a 

 detached series of the left row no. 1 (fig. 10), and these are, again, 

 smaller (measuring 0*016 by 0*012) • but it is not possible to distin- 

 guish with certainty any of the teeth originally placed beyond. 



Exactly as in the previous specimens — and as is to be observed 

 in all associated groups of Ptychodont teeth — there is no remarkable 

 variation in the ornament of the grinding-surface in different parts 

 of the jaw (except in the small median row), and it is interesting to 

 be able to determine that there is no difference in this respect between 

 the upper teeth and the lower. 



Mr. Willett's specimen also supports the conclusion, based upon 

 the National fossils, that there is no regularity in the plan by which 

 one row of teeth is adjusted to the next, and offers many good 

 examples of the slight adaptive variability in shape. But there is 

 one interesting fact in regard to this adaptation which is not so 

 clearly demonstrated in the majority of specimens, and which is worthy 

 of a passing note, since it affords a method of readily distinguishing 

 " lefts " from " rights." The outer edge of every lateral tooth is 

 more or less regular, being almost straight, and terminating anteriorly 

 and posteriorly in a gentle rounded angle, while nearly all the 

 modifications requisite to ensure the continuity of the dental arma- 

 ture are provided by the variable inner edge, In very many cases 

 the posterior inner angle is produced considerably backwards (e. g. 

 fig. 9) ; but neither this nor the other adaptations appear to affect 

 the extent of the transversely ridged median area. 



A further point of interest in the Brighton fossil consists in the 

 well-marked character of the pressure-scars, produced — like those 

 in the teeth of Proboscidian Mammals — by the forward progress of 

 the dentition during growth. They occur as small polished patches, 

 exposing in section the minute medullary canals with which the 

 dentine beneath the surface is everywhere traversed ; and, as might 

 naturally be expected, they are most prominently shown in the older 

 anterior teeth. Typical examples are shown in figs. 8, 9. . Other 

 teeth in the British Museum exhibit the same feature, though I 

 am not aware of its having hitherto been noted, and scarcely any 

 specimens show it so prominently as the example just described. 



VI., YII. In the light of the foregoing facts, it will be interesting 

 now to attempt a revision of the various specific types of the genus 

 Ptyclioclus, and to endeavour to interpret the numerous associated 

 groups of teeth that are already within the reach of scientific 

 inquiry ; but such will be a work of considerable extent, and it is 

 beyond the scope of the present communication to enter this 

 untouched field. 



It may be worth while, however, to append sketches of the median 

 teeth of two of the species, for comparison with those of P. decurrens ; 

 and the originals of figs. 11 and 12 are such teeth, belonging to 

 P. polygyrus and P. paucisulcatus respectively. In the former 

 (fig. 11) the ordinary median area is reduced to a small conical 



