130 



ME. A. S. WOODWAKD ON THE DENTITION 



liarities of the vascular dentine ; and as such investigations failed 

 to separate the Dipnoan Ctenodus and Ceratodus — and, still more, 

 the Psammodonts and Petalodonts — from the Cestraciontidse, it is 

 not surprising that the3 T were likewise unsuccessful in correctly 

 determining Ptycliodus. Microscopical structure, in fact, seems rather 

 to depend upon function, and not so much on genetic relationship. 



In conclusion, the question arises as to whence Ptycliodus came, 

 and whither its descendants, if any, departed. For, so far as is 

 at present known, not a trace of this generic type occurs in any 

 deposits beyond those of Cretaceous age ; and no one has yet 

 succeeded in discovering a Selachian tooth that is obviously a modi- 

 fication of the form so common in the Chalk. Mantell's Ptycliodus 

 Mortoni, it is true, is suggestive of an approximation to a more 

 ordinary type of tooth, and it is not improbably one of the missing 

 links required ; and a Bohemian example, described by Reuss * as 

 P. triangularis, is also worthy of special note for exactly the same 

 reason. But the progress of palaeontological knowledge is necessarily 

 slow and uncertain, and we are compelled to remain satisfied with 

 the present slight advance, while deferring the solution of these 

 wider problems to a future occasion when still more materials may 

 be available. 



EXPLANATION OF PLATE X. 



Fig. 1. Portion of lower dentition of Ptychodus, sp. (B. M., no. 40056.) 



2. Portion of upper (?) dentition of P. decurrens. (B. M., no. 39134.) 



3. Portion of upper dentition of P. decurrens. (B. M ., no. 38564.) 



4. Portion of upper and lower dentition of P. decurrens. (Collection of 



Henry Willett, Esq., F.G.S., Brighton Museum.) 



5. Tooth of upper median series, found associated with the preceding 



specimen. 



6. Tooth of upper series 1, associated with original of fig. 4. 



7. Teeth of right upper series 3, 4. associated with original of fig. 4. 



8. Tooth of lower median series, associated with original of fig. 4. 



9. Tooth of left lower series 1, associated with original of fig. 4. 



10. Teeth of left lower series 1. 2, associated with original of fig. 4. 



11. Tooth of upper median series of P. polygynis. (B. M., no. p 319.) 



12. Tooth of upper median series of P. paucisulcatus. (B. M., no. 4358.) 



13. Much-worn tooth of lower median series of P. decurrens. (B. M. 



no. 33247.) 



B. M. = British Museum, All the specimens are from the Chalk of Kent and 

 Sussex, and are drawn of the natural size, with the anterior border in each case 

 directed downwards. 



Discussion. 



The Peesident congratulated the British Museum on obtaining 

 the assistance of so promising a young palaeontologist as the author 

 of the present paper. 



Prof. Seeley expressed a high opinion of the paper. The subject 

 was one of much difficulty, as was shown by its having baffled a 



* A. E. Reuss, 1 Die Versteinerungen der bohmischen Kreideformation/ 

 pt. i. (1845) p. 2, pi. ii. figs. 14-19. This species was subsequently referred to 

 Acrodusbj A. Fritsch, ' Reptilien und Fische der bohmischen Kreideformation,' 

 1878, p. 16 fig. 38. 



