PETRIFACTIOx\ OF ECHINODF.RMS 1087 



possession of calcareous dermal plates wholly composed of calcite, 

 which in many groups form a solid test or enclosure for the main 

 mass of viscera within. The plates arc not firmly united with each 

 other, hut they ha\e the power to grow and change form during 

 the life of the individual. A characteristic feature of all the skele- 

 tal parts is their extreme porosity. This is true of the test and 

 the spines of the sea-urchin as well as of the calyx, arms, and 

 stem of the crinoids. The porosity shows in section, and it is also 

 indicated hy the fact that the specific gravity of a recent Cidaris 

 spine, with its pores unfilled hy water, is only 1.46, while the com- 

 pletely calcified spine has a specific gravity of 2.7. The hollow 

 spaces constittite about 43 per cent, of the spine. (Haidinger, 

 Tjh\m-4 -.16 1.) During the life of the animal the pores in the cal- 

 careous tissue are occupied by organic matter (chitin), which is 

 removed by decay after death. Furthermore (Haidinger). each 

 skeletal element of the echinoderm test is composed of an individual 

 crystal, the crystalline axis of which is coincident with the organic 

 axis ; and the new lime which fills the pores crystallizes in con- 

 tinuity with the calcite of the original structures. As a result, per- 

 fect cleavage is obtained in the skeletal parts, each plate or spine 

 having virtually become a perfectly cleaving calcite fragment. The 

 axis of the crystal coincides with the organic axis of the part. 

 (Hessel, Blum.) In crinoid stems the cleavage often shows a rota- 

 tion of axis in the successive joints, so that corresponding cleavage 

 planes make an angle with each other. ( Blum-4:/d7.) Accord- 

 ing to investigations carried on by Dr. A. F. Rogers (21) the 

 twisting is sometimes such as to place the crystals composing the 

 successive joints into a twinning position. At other times it is ir- 

 regular, and again in some species there is no twisting at all. 



Silicification of echinoderms occurs more rarely. When it does 

 occur, beekite rings, while present, are often less pronounced and 

 abundant than in molluscs or brachiopods. (Blum-4 i/pi".) 



Pyrites occasionally replaces echinoderm structures. Thus pyr- 

 itized crinoid stems are not uncommon in some formations. Pyri- 

 tized spines of Cidaris have been recorded from the Oolite of Helgo- 

 land. ( Blum-4 -.202. ) 



Entire specimens of Liassic Pentacrinites are found in the sa- 

 propellutytes of Holzmaden, Wiirttemberg, replaced by iron di- 

 sulphide. 



Cerussite (lead carbonate) has been found as a frequent re- 

 placing agent of crinoid remains in the lead-bearing formations of 

 the department of Kielce, southwestern Russia. These replaced 

 remains commonly have the aspect of crystals of this mineral, with 



