INTRODUCTION 5 



Echinoderma. — All of calcite. 



Poly 20a. — Chiefly calcite. 



Brachiopoda. — All of calcite. 



Lamellibranchia. — Many consist entirely of aragonite, 

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

 Mytilus, Spondylus, Unio, and Trigonia, the inner layer is 

 of aragonite, the outer of calcite. 



Gasteropoda. — The majority are formed of aragonite, 

 but Scalaria and some species of Fusus are of calcite. In 

 some {e.g. Patella, Littorina) the outer layer is calcite. 



Cephalopoda. — Nautilus, Spirula, and Sepia are mainly 

 aragonite, as also were probably the Ammonites. Argo- 

 nauta and the guard of Belemnites are calcite. 



Crustacea. — The shell consists of chitinous material 

 usually containing calcite, and often some phosphate of 

 lime. 



The condition in which fossils occur depends, as we 

 have seen, on their original composition and on the 

 material in which they are embedded. The chief types 

 are the following : — 



1. The entire organism preserved. Occasionally the 

 soft parts of the organism are preserved as well as the 

 skeleton, the whole having suffered very little change. 

 Instances of this are, (a) the woolly rhinoceros and mam- 

 moth found frozen in the mud and ice in Northern Siberia, 

 and (6) insects and plants encased in fossil resin, known 

 as amber, found in the Oligocene beds on the Baltic shores 

 of Prussia and in the Tertiary beds near Cromer. 



2. The skeleton preserved almost unchanged. Some- 

 times when the skeleton alone is preserved, it remains 



