IN THE MUD OF THE LEVANT. 115 



silicified ; but the silica replacing the calcareous 

 cells has a more transparent aspect than that sur- 

 rounding and filling them, as if the organism had 

 been first silicified, and then invested with flint of 

 a less transparent character. Moreover, some of 

 these Rotahaj clearly retain part of their original 

 animal matter, apparently in the condition of 

 Molluskite, as advocated by Dr. Mantell. I have 

 already spoken of my instructive specimen from 

 Plamborough Head. The outer calcareous part 

 is silicified, its outline being comparatively dis- 

 tinct, as contrasted with the darker investing flint. 

 The animal portion has shrunk up within the shell 

 into a smaller compass, still preserving its original 

 brown hue, and affording an almost exact repre- 

 sentation, both as to colour and form, of the animal 

 from the Levant (Fig. 29)- . In this case I can see 

 no room for doubting that at least the colouring 

 matter of the animal membrane is preserved ; and, 

 as in such a texture as this it would be difficult to 

 divide the latter from the former, what ground is 

 there for doubting that the animal membrane is 

 itself present either in the state of Molluskite, or 

 in such close and intimate union with the siliceous 

 matter as to be justly regarded as a silicified ani- 

 mal ? This is clearly no cast of the interior of 

 the shell, filled up with a differently coloured flint 



