Affinities of the Genus Parkeria, Carp. 5 



is that the skeleton of Parkeria was composed originally of 

 carbonate of lime, and that phosphatization, when it has 

 occurred at all, has been the result of secondary processes 

 which have operated subsequently to fossilization. The 

 arguments for regarding the phosphatic condition of the 

 skeleton as superinduced and not original may be briefly 

 summed up as follows : — 



1. The phosphatization of calcareous organisms is a well- 

 known and readily intelligible phenomenon. Thus all kinds 

 of calcareous fossils in phosphatic deposits (as in the Green- 

 sand near Cambridge) are liable to have their carbonate of 

 lime more or less extensively replaced by phosphate of lime. 



2. On the other hand, if we suppose an originally phos- 

 phatic organism to have its phosphate of lime replaced by 

 carbonate of lime, then the latter mineral would certainly 

 appear in the form of crystalline calcite. This is not the 

 case, however, with the purely calcareous examples of 

 Parkeria^ the skeleton-fibre of which is composed of granules 

 of carbonate of lime and not of definite crystals. Mr. Carter 

 has described a specimen in which the skeleton-fibre is com- 

 posed of calcspar ; but I have never personally met with a 

 similar example, and such a condition must be regarded as 

 the result of some secondary change. 



3. In all those specimens of Parkeria which are partly 

 phosphatic while parts are in the condition of carbonate of 

 lime, it is invariably the exterior parts — which, necessarily, 

 are those most exposed to chemical actions originating ab 

 extra — which are phosphatic, while the internal and central 

 portions are those which are calcareous. Moreover, even in 

 the most highly phosphatic portions of such specimens, effer- 

 vescence is produced by weak acids, showing that the original 

 carbonate of lime has not been Avholly removed or replaced. 



4. Thin sections of phosphatized specimens, or of the phos- 

 phatized parts of a specimen, show a more or less imperfect 

 preservation of the minute structure of the skeleton. On the 

 other hand, in specimens in which the skeleton is composed 

 wholly of carbonate of lime the minute structure is exqui- 

 sitely preserved. 



Upon the whole, then, I cannot doubt that the skeleton of 

 Parkeria^ like that of the Stromatoporoids, was originally 

 composed of carbonate of lime. The granules which C(jmpose 

 the skeleton-fibre are not, however, intinitesimally minute, 

 but are, on the contrary, of considerable size and of a sub- 

 crystalline character. A closely similar composition of the 

 skeleton out of large subcrystalline granules is seen in llydrac- 

 tinia circumvestiens^ S. V. Wood, and, very conspicuously. 



