Vol. 67.] AND THEIR DISTRIBUTION IN THE ENGLISH CHALK. 265 



The dimensions in millimetres ! of the principal teeth are as 

 follows : — 



Leu; 1 tli. Breadth. 



Small median tooth 8 C> 



Tooth from first inner paired row 14 12 



Tooth from central row opposite dentition 14 12 



Smallest lateral tooth S 4 



These teeth evidently belong to a young; Ptijchodus. 



PL XIX, fig. 21 represents a median tooth (the only one 

 preserved), belonging to an associated set of thirty-six teeth from 

 Bluebell Hill, Burham. It is evident that this tooth is more elon- 

 gated, though somewhat similar in shape ; and, from comparison 

 with the other teeth in the set, the suggestion arises that age 

 accounts for the differences in shape, size, and development of the 

 grooving. 



The dimensions in millimetres are as follows : — 



Length. Breadth. 



Median tooth 11 8 



Tooth of first inner paired row 27 21 



Tooth of central row opposite dentition 32 23 



PI. XIX, fig. 23 represents the only preserved median tooth in 

 another associated set of twenty -three teeth, also from Bluebell 

 Hill. 



The dimensions in millimetres are : — 



Length. Breadth. 



Median tooth 18 11 



Tooth of first inner row, opposite dentition 30 20 



Tooth of central row, opposite dentition 32 25 



PI. XIX, fig. 22 includes two median teeth united to inner 

 paired rows by iron pyrites, and they belong to the set figured in 

 PI. XVII, fig. 1 a. They are 14 mm. long and 9 mm. broad. 



PI. XIX, fig. 24 represents one of two isolated teeth from 

 Holborough, which evidently belonged to the largest form of Pi. de- 

 eurrens; it is 21 mm. long and 12 mm. broad. 



The fine Brighton specimen described and figured by Dr. A. Smith 

 Woodward 2 may be regarded as a typical example of the species ; 

 and I have discovered a few median teeth of the upper jaw of 

 the same form in the zone of Holaster mbglobosm (PL XIX, 

 fig. 22). 



The foregoing sets belonged to individuals which possessed small 

 teeth, the largest being only 14 mm. in length and 10 mm. in 

 breadth, compared with others whose corresponding teeth are 

 51 mm. long and 36 mm. broad. 



Besides these, there are still more marked varieties which have 

 already received specific names, and I now proceed to describe 

 them. 



1 Where measurements are given, the length is reckoned parallel to the 

 grooving, and the width from the anterior to the posterior portion of the 

 tooth, except in the case of the small median teeth, in which case they are 

 reversed. 



2 Q. J. G. S. vol. xiiii (1887) pi. x, fig. 2. 



