276 THE TEETH OE PTYCHODUS AND THEIR [May I9I I, 



Fig. 7. Lateral end tooth of Ptycliodus mortoni Ag., from Kansas (U.S.A.). 



Slightly less than the natural size. 

 Fig. 8. Tooth of Ptychodus mortoni Ag., in the Oxford University Museum, 



from the Chalk near Winchester. (Photograph from drawing in 



Proc. G-eol. Assoc, vol. xiii, 1893-95, pi. v, fig. 4.) 

 Fig. 9. Terminal tooth of Ptychodus polygyrus Ag., belonging to the set shown 



in fig. 3 of PL XXI. Naturalize. 



Discussion. 



Mr. E. T. Newton said that he was glad of the opportunity of 

 testifying to the great value of the palceontological work which the 

 Author had been carrying on for a number of years, and was gratified 

 to think that at length ati important piece of it was likely to be 

 published. Although Ptychodus teeth from the Chalk were so 

 commonly to be seen in collections of fossils, hitherto little had 

 been known as to the horizons from which they came ; but now the 

 Author, by careful collecting, had ascertained both the horizons and 

 the localities from which each species was obtained. Then again, 

 as regarded the arrangement of these teeth in the fish's jaws, very 

 little was known — although Dr. Smith Woodward published some 

 valuable information about this some years ago. The present 

 Author, however, had obtained several examples with a number of 

 teeth in their natural relations ; and his study of these had largely 

 extended the bounds of knowledge, and enabled him to show the 

 wide variation in the form and pattern of the teeth contained in the 

 mouth of one individual fish. The speaker congratulated the Author 

 on the accomplishment of an extremely valuable and interesting 

 piece of work. 



Mr. W. Mtjrton Holmes said that the group of teeth of Ptychodus 

 polygyrus found by him was obtained from the upper part of the 

 Micraster cor-anguinum Zone in the railway-cutting close to 

 Ban stead Station, about 24 feet below the surface. 



Dr. A. Smith Woodw t ard remarked on the exhaustive character of 

 the Author's researches, and considered that he had now placed 

 the naming of the teeth of Ptycliodus on a satisfactory basis. The 

 speaker regretted that there was still no clue to the predecessors of 

 the genus, but was much interested in the Author's observation that 

 the feebly-ridged teeth of Pt. rugosus occurred only in the Upper 

 Chalk. He thought that recent discoveries still favoured the idea 

 that Ptycliodus was a forerunner of the Myliobatidse and Trygonidse, 

 and alluded to the grooves just becoming evident in the roots of the 

 teeth of Apocopodon, found in the uppermost Cretaceous rocks of 

 Brazil and Africa. 



Mr. G. W. Young confirmed the Author's statement as to the 

 infrequency of the finding of Ptychodus teeth by the collector him- 

 self, compared with their abundance in museums. Ptychodus was 

 not really a common fossil, but its teeth having a conspicuous 

 colour, definite shape, and high lustre, readily caught the eye of the 

 quarryman, who preserved every specimen that he saw. When 

 purchasing specimens it was necessary to enquire carefully whether 



