Vol. 67.] AND THEIR DISTRIBUTION IN THE ENGLISH CHALK. 275 



Plate XX. 



Fig. 1. Part of an associated set of teeth, arranged to show a portion of the 

 upper dentition of Ptychodus dixoni, sp. nov. ; from the zone of 

 Terebratulina gracilis, Ouxton, near Rochester. Slightly more than 

 two-thirds of the natural size. (B.M. P 10260, G. E. Dibley Coll.) 



Fig. 2. Large tooth of Ptychodus dixoni belonging to the lower dentition. (This 

 tooth forms part of an associated set of fourteen teeth, the majority 

 comprising the large^eeth of this dentition.) Natural size. In the 

 possession of Mr. J. G. Wilson, Cuxton. 



Fig. 3. Part of an associated set of teeth of Ptychodus latissimus Ag., ar- 

 ranged to illustrate part of the lower dentition. From the zone of 

 Ho/aster planus, Bluebell Hill, Burham, near Rochester. (G. E. 

 Dibley Coll.) 



Fig. 4. A tooth from the central row of the above. Natural size. 



Fig. 5. A tooth from the central paired row of opposite dentition, and belonging 

 to the same set. Natural size. 



Plate XXI. 



Fig. 1. Part of an associated set of teeth of Ptycliodus poly gyrus Ag., high 

 zonal form, arranged to illustrate part of the upper dentition. 



Fig. 2. Part of the same set, to illustrate the lower dentition. Note the varia- 

 tion in development of the grooving of the enamel. The biggest 

 tooth of the central row shown in this figure measures 7 centi- 

 metres in length and 6 - 5 cms. from back to front. From the zone 

 of Micrasier cor-anguinum, Banstead (Surrey). (B.M. P 10771.) 



Fig. 3. Part of an associated set of teeth of Ptychodus polygyrus Ag., low 

 zonal form, nearly flat and showing broad marginal granulation. 

 Zone of Ehynchonella cuvieri (upper portion), Dunton Green. The 

 largest tooth is 5*5 centimetres long and 4 cms. broad. (G. E. 

 Dibley Coll.) Half of the natural size. 



Fig. 4. Three teeth of Ptychodus polygyrus var. marginalis (Ag.). The biggest 

 tooth is slightly less than natural size, and is from the zone of 

 Terebratulina gracilis, Hailing, near Rochester. The two smaller 

 belong to an associated set of teeth from the same zone, Bluebell 

 Hill, near Rochester. (G. E. Dibley Coll.) 



Plate XXII. 



Fig. 1. Three teeth belonging to an associated set of 129 of Ptychodus poly- 

 gyrus Ag., having a flat surface and the grooving gyrate. From 

 the zone of Terebratulina gracilis, Wouklhain, near Rochester. 

 Slightly less than natural size. (B.M. P 10464, G. E. Dibley Coll.) 



Fig. 2. Part of an associated set of teeth of Ptychodus polygyrus, Ag., with the 

 grooving nearly parallel. From the zone of Terebratulina gracilis, 

 Bluebell Hill, Burham. (G. E. Dibley Coll.) Five-eighths of the 

 natural size. 



Fig. 3. Part of an associated set of teeth of Ptychodus mammillaris Ag., from 

 the zone of Terebratulina gracilis, Cuxton, near Rochester. Half of 

 the natural size. (G. E. Dibley Coll.) 



Fig. 4. Part of an associated set of teeth of Ptychodus mammillaris belonging 

 to the edge of the palate. Zone of Holaster planus, Cuxton. 

 Slightly more than half of the natural size. 



Fig. 5. Tooth of Ptychodus mammillaris Ag., low zonal form. Zone of 

 Ehynchonella cuvieri, Betchworth (Surrey). Two-thirds of the 

 natural size. 



Figs. Ga&Gb. Part of an associated set of teeth of Ptychodus rugosus Dixon. 

 Zone of Micraster cor-testitudinarium, Purley. The terminal tooth 

 of the above set is enlarged to nearly three-quarters of the natural 

 size (fig. 6 /;), and is from the base of the Micraster cor-anguinum 

 Zone, Chatham. (G. E. Dibley Coll.) 



