404 



ME. ¥. A. BATHEK ON PETAL0CEINT7S. [^^^g' 1898, 



imist have been a pure or nearly pure limestone. Whether this 

 subsequently became dolomitized, as was the case with most of 

 the Niagara Limestone of Illinois, Iowa, and Wisconsin, cannot be 

 determined from the hand-specimens examined, since they show no 

 trace of dolomite-crystals, even in thin sections under the micro- 

 scope. It seems more probable that the first change was due to 

 silicification, in the form of a gradual molecular replacement of the 

 calcareous stereom^ of the constituent fossils, beginning with the 

 outer layer of each ossicle or fragment. Thus, in the case of a 

 crinoid stem, the silicifying fluid bathed the outside of the stem, 

 the walls of the axial canal from which the soft contents had 

 previously decayed, and the narrow interstices between the 

 columnals formerly filled with ligament. The supply of silica then 

 decreased, and acidulated water passing through leached out any 

 remaining stereom from the inner portions of each fragment. There 

 were thus left a number of hollow boxes, as it were, formed of silica. 



Fig. 2. — Microscope-section of matrix of Petalocrinus mirabilis, 

 specimen y {Cut hy Mr. F. Chapman. Brit. Mus. E6635.) 

 c 



X 30 diam. 



[The drawing is diagrammatic, especially in so far as the crinoid-fragments 

 have been selected from different parts of the original preparation ; it repre- 

 sents the section as seen by polarized light, nicols crossed, a, columnal 

 cut longitudinally, showing axial canal ; b, brachial cut transversely ; 

 c, brachial and covering-plates cut transversely ; d, chalcedony; e, columnal 

 cut transversely. Between all these, and in the cavities formed by solution, 

 are minute crystals of quartz, while minuter crystals are in the meshes of 

 the stereom, which also is silicified. The black areas represent spaces. 

 The brachials certainly, and the columnals possibly, do not belong to 

 Petalocrinus itself.] 



^ Stereom, any hard tissue, forming skeletal structures in Metazoa 

 Invertebrata and in Protozoa: 'Nature,' vol. xliii (Feb. 12th, 1891) p. 345. 



