FOSSILS OF THE O'OLITE. 



G3 



which we find in the upper part of the series, the Te; ehra'tula 

 Thurmanni (Jig. 100), and the Ferebrtfttila impressa (Jig. 101). 

 The moulds of these shells are frequently silicious, and we find, 

 in the upper layers, beds of silicious balls of loose texture, some- 

 times enclosing silicious moulds of shells. 



54. The upper group of the middle o'olite, called coral o'olite, 

 consists almost entirely of limestone ; it is divided into different 

 thick layers, which are distinguishable from each other by their 

 structure. The first or lowest layers are ordinarily compact, grey- 

 ish or yellowish, filled with polypa'ria or corals of a sac'charoid 

 structure, or those which have passed to the silicious state : this 

 constitutes the coral rag of English geologists. Some of the 

 succeeding layers are o olitig, frequently of large irregular grains, 

 mingled with fragments of rolled shells ; others are compact, pars- 

 ing into chalk or even marl of greater or less solidity. 



55. The numerous polypa'ria contained in this group present 

 to us caryophy Ilia (fig. 21), a'strea, meandri'na, madrepores of a 

 great number of species, resembling more or less those of coral 

 reefs, and a great many other genera. Among the shells, ammo- 

 nites are less common ; but above the o'olite, where all the organic 

 remains are broken, the lowest layers contain a great quantity of 

 various shells, among which are neri'nea (Jigs. 102, 103). The 



Fig. 102.- Neri'nea Goodhallii. 



Interior of the shell, shmniwg the 

 plica of its columhella. 



Fig. 103. Neri'nea mosa. 



superior errata contain a great quantity of astartes (figs. 104. 

 105), the most characteristic of which is the astarte minima. 



54. What are the characters of the upper group of the middle o'olite ? 

 What is coral rag ? 



55. What genera of corals are found in the middle o'olite ? What fossil 

 hells do we find in this group ? 



