264: 



MR. G. E. D1BLEY ON THE TEETH OP PTYCIIODCS [May IQI 1, 



The rows diminish in size from the inner pair outwards. In 

 Pt. decurrens there are not more than fifteen rows, while in Pt. mor- 

 toni there are seventeen rows of teeth. It is now, therefore, possible 

 to interpret the numerous sets of associated though scattered teeth 

 of other species, and to determine the variations which occur in 

 different parts of the jaw. 



I shall treat the several species in the order of their appearance 

 in the English Chalk, and utilize the new material in denning them 

 more exactly than earlier observers were able to do. 



Description of the Fossil Teeth. 



1. Ptychodus decurrens Agassiz. (PI. XVII, fig. 2; PI. XIX, 

 figs. 20-24.) 



1839. L. Agassiz, ' Poissons Fossiles,' vol. iii, p. 154- & pi. xxv b, figs. 1, 6, 7, 8, 21 

 {noil figs. 2-5). 



{Specific characters. — Central part of crown of tooth not 

 sharply raised, as a rule gently rounded or flattened. Transverse 

 ridges small and numerous, extending to the lateral ridges and 

 leaving space for a minimum of granulation ; these ridges are 

 sometimes bifurcating or intercalated, ending abruptly or curving 

 sharply on reaching the margin, seldom showing a concentric 

 arrangement ; also rarely subdivided into tubercles. Numerous 

 fine -ridges extend at right angles to the transverse ridges, over 

 the anterior and posterior portions of the tooth ; this is a specially 

 distinctive feature. 



Variations. — The four naturally arranged sets of teeth already 

 described by Dr. A. Smith Woodward differ much from each other, 

 and illustrate well the difficulty of determining isolated teeth. In 

 the first specimen ] the fineness of the transverse ridges and grooves 

 is noteworthy, and the crown in the lower median teeth is only 

 slightly raised. In the second specimen (op. tit. fig. 2) the rela- 

 tively great extent of the granulated marginal area is unusual, and 

 the lateral teeth exhibit curious oblique distortion. In the third 

 specimen (fig. 3), which belongs to the upper jaw, the small median 

 teeth are peculiar in being nearly square, and the large teeth of the 

 inner paired series have a remarkably raised crown. 



In my own collection I have an associated set of eighty-four teeth 

 from Wouldham which are identical with the foregoing, but the set 

 possesses an additional feature in that it contains, among others 

 of the lower jaw, one tooth of the median row which is charac- 

 terized by an unusually elevated crown. This set contains seven 

 teeth of the upper median row. These small median teeth may 

 vary greatly, though similar in the individual, according to age. 

 In PI. XIX, fig. 20 represents a median tooth in situ belonging 

 to this set. The other six median teeth are identical in shape and 

 other features. 



1 Q. J. G. S. vol. xliii (1887) pi. x, fig. 1. 



