EXPLANATION OF THE PLATES. 



Fio: 12. A part of the dental bone of the lower jaw ; showing- the mode in which the 

 teeth are lodged, and the canals for the distribution of the branches of the infe- 

 rior maxillary nerve : p. 109. 



Fig. 13. Transverse section of an adult tooth of I. communis ; showing the filling up 

 the interior cavity by the ossification of the pulpy substance: p. 107. 



Plate XVI. 



Figs. 1 to 5. A series of transverse sections derived from the anterior portion of a 

 large jaw of I. platyodon : p. 112. 



Fig. 6. Side view of the same jaw : ibid. 



Fig. 7. Inferior view of the same ; the posterior part restored from another indivi- 

 dual of the same species. 



Fig. 8. Posterior part of a large head of I. communis belonging to the Oxford Mu- 

 seum : the lower jaw is described p. 112; the parts surrounding the tympanal 

 cavity, p. 116 ; and the occipital bones, p. 117. 



Figs. 9 to 13. Transverse sections of the lower jaw, derived from the preceding spe- 

 cimen : p. 112. 



Fig. 14. Internal view of the posterior part of the lower jaw, from the same speci- 

 men: p. 111. 



N .B. In the text this is erroneously described as the jaw of the left side ; being 

 in fact the right. 



Plate XVII. 



Head of Ichthyosaurus intermedins ; a matchless specimen belonging to Mr. De la 

 Beche. The sternal parts of the same specimen have been figured in the Philoso- 

 phical Transactions for 1819 — Plate 14. This specimen is referred to through- 

 out the description of the bones forming the head in the Ichthyosaurus ; and 

 more especially for those exhibited in the roof of the mouth : see p. 117. The 

 dislocation of the parts connected with the tympanal and occipital portion is ex- 

 plained in the note, p. 116. 



Plate XVIII. 



Three views of the dental bone of the lower jaw in the Plesiosaurus : p. 119. 



Plate XIX. 



Figs. 1 & 2. An entire head of Plesiosaurus, under two points of view : pp. 120, 121. 



Fig. 3. A detached tooth : p. 120. 



The middle figures, A & B, exhibit an attempt to restore the head : see p. 121. 



Plate XX. 

 Fig. 1. Paddle of Ichthyosaurus : p. 118. 

 Fig. 2. Three views of the lower occipital bone, containing the condyle of the Ich- 



