PLATE LI I. 



Fig. Page. 



1-5. Ptychodus decurrens, Agassiz; five associated teeth, crown-views and 



(la— 5a) outline-sections, nat. size. — English Chalk. Bowerbank 



Collection (B. M. no. 39436). 242. 



6. Ditto; imperfect vertebral centrum associated with the preceding teeth, 



nat. size, a., outer smooth face of primitive double-cone ; b., inner 

 face of the same, showing delicate radiating ridges; c, concentric 

 lamina 1 within ; d. } vacant spaces originally occupied by cartilage 

 continuing base of neural or haemal arch. 228. 



7, 8. Ditto; two associated teeth, crown-views, showing tubercles between 



ridges, nat. size. — Zone of Holaster subglobosns ; Wouldham, Kent. 

 Bowerbank Collection (B. M. no. 39138). 242. 



9—11. Ditto; var. oweni, Dixon; three apparently associated teeth, crown- 

 views and the first in outline-section (9 a), nat. size. — Zone of 

 Holaster subglobosus ; Hailing, Kent. Bowerbank Collection (B. M. 

 no. 39125). 242. 



12-15. Ditto; four associated teeth, crown-views, and the first in outline- 

 section (12 a), nat. size. — Zone of Holaster subglobosus ; Holborough, 

 near Rochester, Kent. G. E. Dibley Collection (B. M. no. P. 

 10201). 244. 



16. Ditto; median section of vertebral centrum, showing concentric 

 laminee and spaces for cartilage at the bases of the neural and haemal 

 arches (d), two thirds nat. size. — Chalk; Kent. Mrs. Smith's 

 Collection (B. M. no. 49016). 228. 



