24 



MV GARDEN. 



Besides these traces of marine animals, the flints which we use in 

 the garden have frequently the stems of encrinites attached to them, 



Depressed form. Normal form. 



Fig. 27. — Galerites albo-galerus. 



Fio. 28.— Flint cast of 

 Holaster pillula. 



and the allies of cuttle-fish are to be found in the belemnitella (fig. 30). 

 Occasionally in the chalk is found fossil wood, probably of a coniferous 

 tree, so that there must have been land on which the tree grew when 

 the chalk was being formed under water (fig. 31). 



The beds of flint appear to have an animal origin, and the perse- 

 verance of Mr. Bowerbank proved to geologists at last that the greater 

 part were derived from sponges. A layer of animal matter seems 



FxG. 29. — Pseudo Diadema 

 variolare. 



Fig." 30. — Belemnitella. 



Fig. 5*; — Coniferous wood on flint, 

 with piece enlarg^ed, showing struc- 

 ture and the coniferous discs. 



to have covered the bottom of the sea, and attracted to itself 

 silica or flint. Sometimes the flints extend for miles and miles in one 

 almost continuous agglomerated mass, at other times the flints are 

 more separated. One of these sponges, Polypothecia, from the interior 

 of a flint, is here shown (fig. 32) ; also Ventriculites radiatus (fig. 33). 

 Sometimes the sponge enveloped an echinus or bivalve shell, as 

 sponges do novf—Spondyliis spinosus (fig. 34) and Diadema (fig. 35). 



Sometimes one sponge has been found to interpenetrate another, 

 and both have been silicified together. .Mr. Charles Tyler, of 

 Holloway, possesses a i-emarkable and interesting collection of silicified 



