AND APPINITIES OF PTYCHODUS. 



129 



and the symphysis (as usual in these fishes) produced no line of 

 demarcation in the enveloping dental armature. 



This approximate determination of the affinities of Ptychodus, 

 indeed, seems to furnish a means of deciding upon the relative 

 positions of the two jaws already described. For on examining 

 recent Eays it will be observed that the contours of the dentition 

 are nearly always slightly wavy, with a prominence at the symphysis 

 of the lower jaw, and a corresponding groove in the upper : and 

 this relation appears to be quite constant throughout the group. 



There can be little doubt, therefore, that the lower jaw of the 

 extinct genus under consideration was armed by the teeth so well 

 displayed in the original of fig. 1, while the upper was provided 

 with- the series most satisfactorily shown in fig. 4. 



But with regard to the family with which the Ptychodonts ought 

 properly to be associated, the dentition alone is insufficient to afford 

 a definite clue. For among recent Rays the characters of the teeth 

 are never of more than generic value * ; and there is so much 

 variation among the members of the same group that it is impossible 

 to proceed beyond a hazardous speculation. Even so peculiar a 

 dental arrangement as that of Myliobatis, for example, does not 

 prevail throughout the entire family to which it belongs, since one 

 genus (Dicerobatis) appears totally different and is armed with 

 ordinary diminutive teeth, while another (Ceratoptera) possesses no 

 dentition whatever in the upper jaw, and has nothing of importance 

 in the lower f . Owen and others, as already remarked, have fre- 

 quently noted the superficial resemblance of Ptychodus to the living 

 ffliynchobatus, and it is quite possible that there may be some 

 natural affinity ; but in this form the teeth are diamond-shaped, 

 with the transverse ridges extending from corner to corner ; and 

 their quincuncial arrangement and microscopical structure are 

 serious obstacles to a belief in the suggested relation just mentioned. 

 The arrangement of the teeth in parallel rows, crossing the rami at 

 right angles, and their gradual diminution in size from the median 

 series outwards, are features perhaps indicating some affinity with 

 the huge Myliobatidae ; and it is in proximity to these that I would 

 venture to assign the genus a place. Unfortunately, however, the 

 broken fragments of cartilage that constitute the only part of the 

 skeleton hitherto discovered are not sufficiently perfect to yield any 

 more definite information ; and it is quite possible that we are here 

 concerned with a representative of an extinct family as yet unknown 

 to biological science. 



And, in arriving at such a result, it is impossible to omit a passing 

 allusion to Prof. Sir Richard Owen's supposed decisive argument 

 in a different direction, derived from a study of microscopical 

 structure. The distinguished author of the 4 Odontography ' 

 himself has doubtless long ago abandoned the idea of being able to 

 recognize fossil Cestraciont teeth from an examination of the pecu- 



* A. Giinther, 'Catalogue of Fishes in the British Museum,' vol. viii. 

 pp. 434-498. 



t A. Giinther, op. cit. pp. 496-498, 



