AGE OF REPTILES. 



255 



tries where chalk abounds, flint is foimd in it to a con- 

 siderable depth. Sometimes it appears in layers, but 

 commonly in nodules or lumps, of various sizes and 

 shapes. The nodules are sometimes irregular and gro- 

 tesque, and when so are quite a favorite ornament in cot- 

 tage gardens in England. Covered with whitewash, 

 they are used as edges to the borders. One of these 

 forms you see in Fig. 154. Sometimes these nodules 



Fig. 154. 



take such a form as to be called fossil mushrooms. In 

 such cases the flint was depos- 

 ited upon and in certain com- 

 pound polypes of these forms, 

 or, in other words, they are pet- 



t f \ h BJ rifactions of the frame-work of 



these animals. Two of these are 

 seen in Fig. 155. Such forms 

 are called Ventriculites. 



362. Fossils in the Flint. There are some very inter- 

 esting minute fossils in the flint nodules. Some of these 

 I will notice. There are certain vegetable fossils called 

 Xanthidia, which, like the diatoms, were formerly sup- 

 posed to be animal. There are many species of them. 

 In Fig. 156 (p. 256), at 2, you see represented a chip of 

 flint, which is so thin that the Xanthidia can be seen by 

 transmitted light. The five spots mark the frame-works 

 of five of them. The figure below exhibits one of these 



