LIFE. 609 



mentioned by Barrande of the conversion of the animal material within a 

 Lower Silurian Orthoceras into adipocere (an animal substance having the ap- 

 pearance of spermaceti), and he speaks of them as the oldest mummies ever 

 exhumed. 



A small percentage of phosphates and fluorids is derived from decomposing 

 animal tissues. 



The Excrements of animals afford a considerable amount of phosphates, and, 

 by decomposition, ammoniacal compounds. The latter are dissipated mostly in 

 the air, or by solution in waters ; while the phosphates are often distributed 

 through the earth in which the animals live, or else are accumulated in beds, as 

 in the case of guano. The excrements of the larger animals retain their form, 

 and constitute the fossils called Coprolites. The amount of phosphates from the 

 life which swarms in some muddy sea-bottoms and shores must be large. For 

 analyses of Coprolites, see page 67. 



Bones are combined with so large an amount of animal gelatine that they are 

 the food of various animals ; and this is a great source of their destruction. 

 Again, when the animal matter decays, the bones are left very fragile, unless 

 hardened anew by a substitution of mineral matter. In the Cartilaginous fishes, 

 the backbone, when it fails wholly of stony material, is not found fossil, as in 

 most fossil Ganoids. 



The teeth of Vertebrates contain much less animal matter than bones, and 

 also a coating of enamel, in which there is considerable phosphate of lime. 

 They are therefore exceedingly durable, and the most abundant of the remains 

 of many species. The bony enamelled scales of Ganoid fishes are equally en- 

 during, differing much in this respect from the membranous scales of Teliosts. 



Of Shells and Corals analyses are given on page 66. As the amount of animal 

 matter present is usually very small, they have great durability. 



A few shells, as those of the Lingulse and Oboli, and probably those also of 

 Pteropods, contain, like bones, a large amount of phosphate of lime (p. 68). 



Traces of phosphates and fluorids are present in both shells and corals. 



In a few rare species of Coral of the Gorgonia family, the stony secretions 

 are siliceous. The Polycystines are siliceous Protozoans. 



The siliceous shells of Diatoms and spicula of Sponges have been an import- 

 ant source of silica in rocks of all ages (pp. 68, 271, 482, 488). This silica is 

 usually what is called soluble silica (pp. 55, 488). The index of refraction, as 

 determined by Rood, from Diatoms is only 1.435, while that of ordinary quartz 

 is 1.548. 



The material of the sponge also is sometimes siliceous, though generally more 

 like horn in nature. It becomes filled in with mineral matter, and in this state 

 forms the fossil sponge. A few species of sponge have calcareous spicula. 



(b.) The fitness of species for becoming fossilized or concreted 

 into rocks depends in part on their place and habits of growth. 



Water-species of plants and animals are those most likely to be- 

 come fossils and contribute to rock-formations ; and next those 

 that live in marshes, or along shores or the borders of marshes. 

 The reasons are two: — (1) Because almost all fossiliferous rocks are 

 of aqueous or marsh origin ; and (2) because organisms buried under 



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