AND AFFINITIES OF PTTCHODUS. 



123 



Descriptions of Specimens. 



The detached teeth of Ptycliodus are so familiar and have been so 

 frequently figured, that nothing remaius to be added to the pub- 

 lished descriptions of their general form. But it will be convenient 

 to anticipate slightly by referring to their original relations in the 

 jaw. Each tooth was so placed that the large characteristic ridges 

 and furrows had a transverse direction, while the borders parallel to 

 these were completely anterior and posterior. Judging from the 

 analogy of several recent Selachians to which the genus is most 

 nearly allied, the somewhat excavated and abrupt boundary of the 

 crown was posterior, while the more gently sloping and overhanging 

 border formed the front ; this is the interpretation adopted here, and 

 in all our figures the anterior border is directed downwards. 



On examining the associated groups of teeth, it at once becomes 

 evident that they were arranged in the mouth on two distinct, but 

 yet closely similar plans. The first type, of which I have already 

 published a sufficiently accurate diagrammatic sketch (loc. tit.), is 

 admirably elucidated by the fossils in the British Museum ; and 

 the second is equally well shown in Mr. Willett's specimen, which, 

 moreover, demonstrates that the two arrangements were opposed to 

 each other in the dentition of the same individual. 



I. An interesting forerunner of the fossils revealing the first of 

 these plans is a small example of P. decurrens, figured many years 

 ago in Dixon's ' Geology of Sussex ' (plate xxxii. fig. 5) ; but this is 

 scarcely complete enough to give any satisfactory clue to the entire 

 armature of which it formed a part. Only nme teeth are shown, 

 though these indicate portions of no less than four parallel rows, 

 one represented by three components, and the remainder exhibiting 

 but two. There is evidence of one series composed of relatively 

 large teeth, and the rows immediately adjoining this on either side 

 are so completely alike in character and dimensions as to appear 

 quite symmetrically disposed in regard to it ; while a still more 

 diminutive series alone remains to the left. 



II. But the specimen of greatest interest in this respect still re- 

 mains undescribed, and is shown of the natural size in PI. X. fig. 1. 

 Its precise locality is unknown, and there is some difficulty in its 

 specific determination : but it is evidently a close ally of P. decurrens, 

 even if not a variety of that form. Thirty-four teeth have been 

 disengaged from the surrounding chalky matrix, and of these no 

 less than twenty-three are shown in their natural order. There are 

 remains of six parallel series, and the general appearance of the 

 specimen is at once suggestive of symmetry about the row of largest 

 teeth, which would thus be median when the dentition was com- 

 plete. This series (o) is represented by five of its components, 

 a ad it is noteworthy that there is scarcely any variation in size as 

 they are followed from behind forwards ; all, indeed, have approxi- 

 mately the breadth of 0-021 m., and the length of 0-016 m * The 

 series (i) immediately to the left of the median row is likewise 



* All the measurements are given in decimal fractions of a metre. 



K2 



