ll'iJ The Rev. \V. D. Conybeare's Additional Notices 



Before quitting the lower jaw^ I wish to call the attention of the reader to 

 two series of transverse sections^ exhibiting the arrangement of its boncs^ from 

 very perfect specimens. See figures 1. to 5. and 9. to 13. Plate XVI. 



A similar series was given in the former memoir^ already referred to ; but 

 had the disadvantage of being made up from different specimens. The pre- 

 sent, being deduced from two individuals only, illustrates this subject with 

 more certainty ; since a variation, arising from difference of species or of age, 

 might be suspected in the former instance. The specimens now figured are 

 also remarkably free from compression. 



The first of these series (figs. 1. to 5.) is taken from the anterior portion of a 

 very large lower jaw of ichthyosaurus platyodon, in the possession of Mr. De 

 la Beche ; of which a lateral view is given in fig. 6 : and the bottom is repre- 

 sented, with its hinder extremities restored, from another specimen, in fig. 7. 

 The second series (figs. 9. to 13.) includes only the posterior half of the lower 

 jaw, and is taken from the very fine specimen of ichthyosaurus communis be- 

 longing to the Oxford collection, which is shown entire at fig. 8. PI. XVI. 

 The section, fig. 4. nearly corresponds, as to the place of the line in which it 

 traverses the jaw, with that marked 13. ; and fig. 5. with fig. 12. ; so that the 

 slight differences which exist, may be considered as characterizing the diffe- 

 rent species. 



In comparing these with the figures given in the former memoir. Vol. V. 

 PI. 40. it will be observed, — that whereas in the specimens of I. communis and 

 I. platyodon now figured, the coronoid bone disappears on the outside (being 

 overlaid and concealed by the overhanging flap of the dental) before the similar 

 concealment of the angular bone, — in those formerly figured the angular draws 

 itself up beneath the coronoid, before the coronoid is thus covered up itself. 

 The latter structure is apparently characteristic of the species I. intermedius, 

 and will be seen well displayed in the lower jaws of the beautiful specimen 

 figured in PI. XVII. 



Pig. 7. of the sections in the first memoir (Vol. V. PI. 40.) appears to require 

 correction, where the dental bone, u, is represented as ending in a thick knob. 

 An accidental fissure must in this instance have been mistaken for the true su- 

 ture between the bones, since in every other instance the dental expires in a 

 thin plate. 



I would particularly invite attention to the beautiful manner in which these 

 bones appear to be packed and adjusted together in the series now figured. 



Another striking contrivance in the structure of this composite lower jaw de- 

 serves to be noticed. Where the coronoid is interposed between the dental and 

 opercular, its fibres have a slanting direction ; while those of the two latter bones 



