26 INTRODUCTION. 



5. Annelida. — The skeleton seems to be always composed of calcite. 



6. Crustacea. — The shell of the Crustaceans is mainly composed of 

 calcite, with a variable intermixture of phosphate of lime. 



7. Polyzoa. — The skeleton of the calcareous Polyzoa consists of a vari- 

 able intermixture of calcite and aragonite, the two inseparably blended 

 together. 



8. Brachiopoda. — The shell appears to be always composed of calcite 

 [sometimes with a considerable proportion of phosphate of lime]. 



9. Lamellibranchiata. — The shell of the Bivalve Molluscs is often com- 

 posed wholly of aragonite, but in other cases {e.g., Oysters and Scallops) 

 it is wholly of calcite, while in others {e.g., in Mussels, Pinna, &c.) the 

 shell has an outer layer of calcite and an inner layer of aragonite. 



10. Gastropoda. — Most Univalve Molluscs have the shell wholly com- 

 posed of aragonite, but some (such as Patella, Fusus, Littorina, and 

 Purpura) possess an outer layer of calcite and an inner layer of 

 aragonite. 



11. Cephalopoda. — The shells of Cephalopods appear to be mainly 

 composed of aragonite. 



The above-mentioned variations in the chemical composition of the skele- 

 ton of calcareous organisms have been shown by Sorby to be associated 

 with important differences as to the condition of preservation of these 

 skeletons as fossils. It has been shown, namely, that aragonite is rela- 

 tively much less stable than calcite. Calcite has no tendency, under any 

 natural circumstances, to pass into the condition of aragonite ; aragonite 

 very readily passes into the condition of calcite. Hence in the processes 

 connected with fossilisation, calcareous skeletons composed of aragonite 

 are much more liable to undergo alteration, replacement, or even dissolu- 

 tion, than are those composed of the more stable calcite. It is for this 

 reason that the shells of Gastropods and Lamellibranchs — which are 

 commonly composed entirely of aragonite — are so often found as fossils 

 in the condition of mere " casts," the original shell having been wholly 

 removed by solution, or having been replaced by a pseudomorph com- 

 posed of irregularly placed (unoriented) crystals of calcite. For the 

 same reason, in cases where the shell consists in part of aragonite and 

 in part of calcite, it is common for the aragonite layer to have been re- 

 moved, while the calcite layer has been preserved. 



Before leaving the subject of limestones, it may be advisable to 

 notice briefly the more important differences as regards chemical 

 constitution or minute structure which give rise to special types of 

 limestone, and which not infrequently have a palaeontological signifi- 

 cance. The differences here specially alluded to may be considered 

 under the following heads : — 



(1.) Lithological Nature. — Many of the differences which dis- 

 tinguish particular varieties of limestone concern simply the mineral 

 nature of the rock, and are of no special importance from a palaeon- 

 tological point of view. Thus, many limestones are more or less 

 extensively made up of angular quartz-grains embedded in a matrix 

 of crystalline calcite (fig. 2, b), the rock becoming an arenaceous lime- 

 stone. There are innumerable links, in fact, between what may be 

 called a " calcareous sandstone " and a true " limestone " containing 



