PETRIFACTION OF INVERTEBRATES 1083 



films, but cases of pyritization among the graptolites are not un- 

 common. Such pyritized specimens stand out in relief and afiford 

 good material for sectioning. (\Viman-27.) Pyrophyllite also 

 has been found replacing these organisms, which thus became out- 

 lined in white on the dark shales in which they occurred. (Blum-5 : 

 I26\ 7:173.) 



In a considerable number of sponges siliceous or calcareous 

 spicules occur, frequently uniting into a solid network, and then 

 preserving the form of the sponge. The siliceous spicules of 

 sponges are sometimes replaced by calcite in the process of fossili- 

 zation. 



(c) Siliciftcation of corals. ^lost corals are composed of cal- 

 cium carbonate in the form of aragonite, with the exception of 

 the Alcyonaria, which are calcite. Often a small percentage of 

 magnesium carbonate is present. The structure of the corals is 

 frequently very porous, but it is most probable that these pores 

 are first filled by calcium carbonate, and that silicification is a proc- 

 ess of replacement pure and simple. While silicified corals pre- 

 serve the form well, the finer structure is commonly destroyed. 

 The ringed structure, known as Beekite rings and more fully de- 

 scribed under the section on molluscan shells, occurs rarely in 

 corals ; the rings seldom occur so abundantly or of such size as in 

 molluscs or brachiopods. Corals are occasionally replaced by other 

 minerals, sphalerite having been most frequently observed. 



(d) The hrachiopod shell. In a number of inarticulate brachio- 

 pods the shell consists chiefly of chitin, and here the preservation is 

 similar to that of other chitinous structures. In Lingula, alter- 

 nating layers of chitinous and calcareous matter make up the shell, 

 but iji the majority of species the shell is wholly composed of cal- 

 cium carbonate. This is present in the form of calcite. The 

 greater portion of the shell is composed of a layer of fibers or 

 prisms of calcic carbonate which constitutes the inner layer of the 

 shell. Outside of this is a thin lamellar layer of calcic carbonate, 

 covered in turn by the periostracum. or outer corneous film. In 

 a large number of species the shell is traversed by vertical canals 

 or tubules which expand upward and terminate in the lamellar 

 layer, not piercing the periostracum. 



Calcification and silicification occur in the brachiopods as in 

 the mollusc shells, the tubules, when present, forming additional 

 spaces for the infiltration of lime or silica. Details will be men- 

 tioned in the description of molluscs. Nearly all the minerals men- 

 tioned under molluscs have been found replacing hrachiopod shells ; 

 t. e., pyrite, galenite, sphalerite, the various iron oxides, barite, etc. 



