INTRODUCTION 25 



follows the Latin order of names, the family name being placed 

 first and the individual name last. The ancient Romans would 

 not have said John Jones, but Jones John. 



Composition of the hard parts of organisms. — The compo- 

 sition of the hard parts varies considerably in different organ- 

 isms and consequently causes a variation in the preservation 

 of the different forms as fossils. For example, percolating waters 

 dissolve aragonite with great ease, but have little effect upon 

 chitin. 



The following list contains the mineral composition of the 

 hard parts of the more important animal and plant classes : 



Animals. — i. Lime carbonate: Foraminifera (most); 

 sponges (some) ; Hydrozoa (some) ; corals (most) ; MoUus- 

 coidea (most) ; mollusks (most) ; Echinodermata (all). 



a. Calcite (CaCO.3) : Foraminifera (vitreo-calcareous forms, 

 such as Globigerina) ; corals (many) ; sponges (Calcarea) 

 Bryozoa (most) ; brachiopods (all except the phosphatic ones) 

 worms (Serpula, etc.) ; Crustacea (calcareous part of skeleton) 

 Pelecypoda (many entirely of calcite, as Pecten, Ostrea ; many 

 with only outer layer of calcite and inner of aragonite, as Mytilus, 

 Unio) ; Gastropoda (feW; as Scalaria ; some, as Patella and Lit- 

 torina, have only outer layer of calcite) ; Cephalopoda (few, as 

 Argonauta and Belemnites) ; Echinodermata (all) ; hens' eggs. 



h. Aragonite (CaCOg) : Foraminifera (probably most por- 

 cellanous forms, as Peneroplis) ; corals (Madreporaria) ; most 

 scaphopods, pelecypods, gastropods, and cephalopods. 



Calcite and aragonite have the same composition (CaCOs) 

 but crystallize in different systems. Aragonite crystallizes in 

 the orthorhombic system ; in shells it usually has a chalky, 

 opaque appearance and is compact throughout. Calcite crys- 

 tallizes in the hexagonal system ; in shells it is usually translu- 

 cent, with a compact surface but porous interior. In carbonic 

 acid it dissolves more slowly than aragonite and hence when 

 subject to percolating waters persists longer. In Mytilus ediilis, 

 the outer layer of the shell is of calcite, the inner of aragonite ; 



