CALCAREOUS SKELETONS. 5 



Hope's Nose, Torquay, had lost the inner layer but not 

 the outer. In some cases aragonite is replaced by calcite, 

 but then the organic structure is entirely destroyed, and 

 we get merely a mass of calcite crystals. But calcite is 

 never replaced by aragonite. The mineral character of the 

 skeleton of the chief calcareous organisms is as follows : 



Foraminifera. The vitreous forms consist of calcite, 

 the porcellanous probably of aragonite. 



Actinozoa. The Alcyonarians mainly of calcite, but 

 most of the Madreporaria of aragonite. 



Echinodermata. All of calcite. 



Polyzoa. Calcite and aragonite in layers, or sometimes 

 perhaps mixed. 



Brachiopoda. All of calcite. 



Lamellibranchiata. Many consist entirely of aragonite, 

 but Ostrea and Pecten of calcite. In Pinna, Mytilus and 

 Spondylus, the inner layer is of aragonite, the outer of 

 calcite. 



Gasteropoda. The majority are of aragonite, but Sca- 

 laria and some species of Fusus are of calcite. In some 

 {e.g. Patella, Littorina, Fusus antiquus) the outer layer is 

 of calcite. 



Cephalopoda. Nautilus and Sepia are mainly aragon- 

 ite, as also were probably the Ammonites. The guard of 

 Belemnites is calcite. 



Crustacea. The shell consists of calcite, with some 

 phosphate of lime. In some cases there is a large amount 

 of organic matter. 



The conditions in which fossils occur depend, as we 

 have seen, on their original composition and on the ma- 

 terial in which they are embedded. The chief types are 

 the following : 



