274 MB. G. E. DIBLET ON THE TEETH OF PTYCHODUS [May 191 I, 



and enlargements necessary for the elucidation of the subject. 

 Finally, I wish especially to offer my warmest thanks to Dr. A. 

 Smith Woodward, who has not only been my constant friend and 

 adviser throughout the preparation of this paper, but has always 

 inspired and fostered my energies towards unravelling the zonal 

 history of the Fishes of the English Chalk. 



EXPLANATION OF PLATES XVII-XXII. 



Plate XVII. 



Fig. 1 a. An associated set of teeth of Ptychodus clecurrens (Ag.) var. depressus 

 of Dixon; from the zone of Holaster subglobosus, Bnrham, near 

 Rochester, found in situ. The lower portion illustrates the curva- 

 ture of the palate, the whole series belonging to the upper dentition. 



Fig. 1 b. A detached portion of the above set containing three small median 

 teeth ; this group is cemented by pyrite, and is of the natural size. 

 (B.M. P 10336, G. E. Dibley Coll.) 



Fig. 2. An associated set of Ptychodus decurrens (Ag.), found in situ and re- 

 presenting part of the lower dentition ; from the zone of Holaster 

 subglobosus, Merstham (Surrev). Slightly less than natural size. 

 (G.E. Dibley Coll.) 



Plate XVIII. 



Figs. 1-11. A series of teeth preserved in the British Museum (Natural History), 

 hitherto named Ptychodus oweni (Dixon) : to show that they 

 merely represent a phase of extreme variation of Ft. decurrens. 

 (Compare figs. 3, 5, & 8 with figs. 1, 2, 3, & 17 of PI. XIX.) Three- 

 eighths of the natural size. 



Figs. 12 & 13. Part of an associated set of teeth of Ptychodus decurrens, 

 arranged hypothetically to represent (12) part of the upper den- 

 tition and (13) part of the lower dentition. This set also illustrates 

 extreme variation ; the big central teeth were known formerly as 

 Pt. multistriatus (A. S. Woodward). Half of the natural size. 



Plate XIX. 



Figs. 1-3. Three teeth of Ptychodus decurrens (part of an associated set) 

 belonging to the central row of the lower dentition, and illustrating 

 abnormal development of the enamel (figs. 4 & 5 also belong to 

 the same individual, and illustrate malformation). Half of the 

 natural size. From the zone of Holaster subglobosus, Oxted (Surrey). 

 The largest tooth is 5*7 centimetres long and 5 cms. broad. (G. E. 

 Dibley Coll.) 



Figs. 4-19. A series of teeth of Ptychodus decurrens, arranged to show marked 

 variation of the species. From the zone of Holaster subglobosus of 

 various localities. Half of the natural size. (G. E. Dibley Coll.) 



Figs. 20-24. Median teeth of the upper dentition of Ptychodus decurrens 

 belonging to five individuals, and arranged according to age (see 

 p. 265 in regard to dimensions). No. 22 belongs to the set shown 

 in PI. XVII (B.M. P 10336, G. E. Dibley Coll.). 



Fig. 25. Posterior aspect of a tooth of Ptychodus decurrens, showing the fine 

 grooves developed at right angles to the crown and continuous to the 

 root, a great characteristic of this species ; from the 

 zone of Rhynchonella cuvieri, Betchworth (Surrey). (Gr. E. Dibley 

 Coll.) 



Fig. 26. Terminal tooth of one of the lateral rows of a small Ptychodus 

 decurrens bordering the anterior portion of the palate, belonging to 

 an associated set from the zone of Holaster subglobosus, Wouldham, 

 near Rochester, Very slightly enlarged. (G. E. Dibley Coll.) 



