10 



DESCRIPTION OF THE PLATES. 



magnified 400 linear times. Fig. 4. A 

 portion of the middle of the enamel, 

 magnified 700 linear diameters. Origi- 

 nal. 



PLATE 17. 



Fig, i. — Side view of a tooth of the Ptycho- 

 dus latissimus. Fig. 2. Grinding surface 

 of ditto. Fig. 3. Side view of a tooth 

 of the Psammodus magnus. Original. Fig. 

 4. The grinding surface of four teeth of 

 the Strophodus. Agassi*. 



PLATE 18. 



A longitudinal vertical section of the tooth 

 of the Ptychodus decurrens : magnified to 

 the scale shown by the figure of the nat. 

 size. Original. 



PLATE 19. 



Fig. 1. — Part of a longitudinal section of 

 ditto : magnified 230 linear times. Fig. 

 2. Part of a transverse section, magni- 

 fied 500 linear times. Original. 



PLATE 20. 



Half of a vertical section of the tooth of 

 Psammodus magnus : magnified 50 diame- 

 ters. Original. 



PLATE 21. 



The upper portion is the peripheral border 

 of a vertical section of the same tooth, 

 demonstrating the closure of the medul- 

 lary canals by calcification of their con- 

 tents, close to that border ; which process 

 of consolidation is proved by sections 

 of teeth in different stages of abrasion 

 to have gone on progressively with such 

 stages. The lower portion is a transverse 

 section of two medullary canals, near 

 their point of anastomosis : both magni- 

 fied 500 diameters. Original. 



PLATE 22. 



Fig. 1 . — Part of lower jaw with the dental 

 plates of the Cochliodus contortus. Agas- 

 siz. Fig. 2. One of the dental plates 

 of the Ceratodus gibbus . Agassiz. Figs. 

 3, 4 & 5, different views of the teeth 

 of the Petalodus Hastingsia. Original. 

 Fig. 6. Upper surface of part of a tooth 

 of the Ctenodus cristatus. Agassiz. Fig. 

 7. Vertical section of the same. Agas- 

 siz. 



PLATE 23. 



Fig. 1. — Teeth of the upper jaw of the 

 Rhina. Fig. 2. Teeth of the lower jaw 

 of the same. Fig. 3. A single tooth of 

 the same, magnified. Original. 



PLATE 24. 



Fig . 1 . — Longitudinal section of a tooth of 

 the Rhina. Fig. 2. A section of the 

 exterior enamel-like layer, showing the 

 undulated and branched terminations of 

 the dentinal tubes. Original. 



PLATE 25. 



Fig. 1. — Jaws and teeth from behind of 

 an Eagle-ray (Myliobates aquila). Agas- 

 siz. Fig. 2. A portion of the dental 

 series of the upper jaw of the Rhinoptera 

 marginata, showing a variety in the sub- 

 division of the teeth next the right side 

 of the broad middle row. Original. 

 Fig. 3. Side view of a single dental plate 

 of an Aetobates Narinari. Fig. 4. Lower 

 jaw and dental series of Aetobates Nari- 

 nari, from behind. 



PLATE 26. 



Vertical and longitudinal section of a tooth 

 or dental plate of the Myliobates Aquila : 

 a, processes or ridges for the attachment 



