ON THE DENTITION AND AFFINITIES OE PTYCHODTJS. 



121 



8. On the Dentition and Affinities of the Selachian Genus 

 Ptychodtts, Agassiz. By A. Smith Woodwakd, Esq., F.G.S., of 

 the British Museum (Natural History). (Head December 15, 

 1886.) 



[Plate X.] 



Notwithstanding the abundance of the well-known teeth of Pty- 

 chodus in the Chalk of many localities, and the long list of specific 

 forms that have already been recognized, very little information has 

 hitherto been published in regard to the j>recise affinities of the fish 

 to which they originally appertained. So rarely, indeed, are any 

 of the teeth found associated in natural sequence, that it has been 

 necessary to await the results of many years' patient collecting 

 before being able to pronounce a decided opinion • but materials are 

 now forthcoming for at least one further step in the determination 

 of their relationships, and I therefore venture to offer to the Geolo- 

 gical Society some account of the accumulated evidence. 



Since these interesting fossils first became the subject of scientific 

 study, it has been almost the universal custom, as is well known, 

 to refer the genus to the somewhat comprehensive " f amily " of Ces- 

 traciontidaB. An early determination of Mantell *, it is true, resulted 

 in the suggestion that they formed the dental armature of fishes 

 allied to the Teleostean Diodons ; but the elaborate researches of 

 Agassiz f, supported by Owen's simultaneous investigation of their 

 microscopical structure have always been cited as ample proof 

 of the affinity of their original possessor with the Cestraciont Sharks ; 

 and the deeply rooted tendency among palaeontologists to refer every 

 isolated crushing-tooth to the same extraordinary group has also 

 contributed to the adoption of this arrangement without serious 

 question. 



But the original observations from which the inferences as to the 

 systematic position of Ptychodus were deduced are obviously of a 

 very uncertain and inconclusive character. In the absence Of any 

 but scattered remains Agassiz was compelled to make use of sug- 

 gestive appearances and probabilities rather than well-ascertained 

 facts ; and in framing his conclusions he particularly emphasized 

 at least two of these leading points. In the first place, certain 

 groups of teeth seemed to be so arranged that the smaller and more 

 prehensile examples occupied an anterior position, while the more 

 truly grinding-teeth were scattered posteriorly, thus indicating 

 a disposition analogous to that of the living Gestracion ; while, 

 secondly, there were good reasons for suspecting that a number of 



* G. A. Mantell,' 4 Fossils of the South Downs/ 1822, p. 231. 

 t L. Agassiz, ' Eecherches sur les Poissons Fossiles,' vol. iii. pp. 56-59, 

 150-158,162. 



\ R. Owen, " On the Structure of Teeth," Brit. Assoc. Rep. 1838, Trans. 

 Sect. p. 140 ; and ' Odontography/ pp. 57-59, pis. xviii., xix. 



Q.J.G.S. No. 170. ~ E 



