BLUE CHALK MARL. 87 



Fig. 12. A cast in argillaceous ironstone ; the front of the lower whorl 

 is seen in this specimen, but the outer Up is wanting. 



Fig. 14. exposes the lower wreath with its carinated ridge, and 

 spinous process. 



8. Ampullaria canaliculata. Tab. xix. fig. IS. 



Ventricose, whorls, three or four ; transversely and obHquely striated ; 

 the striae decussating each other ; spire short, turns of the spire separated 

 by a deep channel. 



9. Natica? Tab. xix. fig. 31, 32. 



These shells are from NorUngton, but are too imperfect to admit of 

 accurate determination. 



10. Dentalium striatum. Tab. xix. fig. 4. {Min. Conch, tab. Ixx. 4.) 

 Slightly arched, longitudinally striated; striae ten or eleven, aperture 

 circular. 



Is of frequent occurrence at Folkestone, but is very rare in Sussex. 



11. Dentalium. Tab. xix. fig. 28. 



This specimen is longitudinally striated, and much compressed ; it is in 

 too mutilated a state to allow of specific distinction. 



12. Dentalium ellipticum. Tab. xix. fig. 21, 25. (Min. Conch, tab. Ixx. 

 6,7.) 



Nearly straight, shghtly compressed, aperture circular, external edge 

 elliptical. 



The substance of the shell being thinner laterally, than on the an- 

 terior and posterior margins, the external outline of the tube is of an 

 elliptical form, although the aperture is perfectly circular. The annular 

 markings occasioned by the lines of increase are very numerous, and 

 render the surface uneven. The shell is changed into a white pulve- 

 rulent carbonate of lime ; casts of the interior, having a black polished 

 surface, are not uncommon. Mr. Sowerby remarks, " that they are beau- 

 tiful oblong cones, which remain after the shell is decomposed, and have 

 often puzzled collectors, from the difficulty of ascertaining their origin." 



Tab. xix. fig. 21. A specimen in which the shell is preserved. 



Fig. 25. A cast of indurated marl. 



