Hydractinia, Parkeria, ayid Stromatopora. 55 



Since M. Fischer has given no detailed description of his 

 H. cretaceay it is impossible to say if this be the same 

 species. 



Paekeria, Carpenter. (PI. VIII. figs. 18-17.) 



We now come to Parkeria, whose skeleton was formed not 

 of solid material, like that oi Hydractinia calcarea and the two 

 fossil species last mentioned, but entirely of reticulated tissue 

 like that of Chitina ericopsis (Ann. & Mag. Nat. Hist. I. c), 

 out of which the whole structure, architecture as it may be 

 termed, was elaborated without, as before stated, " core or 

 cortex " — in short, somewhat like a mass of " crochet knit- 

 ting" or the woody fibre of a washed-out gourd {Luffa), to make 

 the similes more familiar (fig. 14) — of course supplied with 

 sarcode when living, which completed the cavities indicated 

 by the architectural arrangement. Such, then, having been 

 the tissue, as it may be termed, and the structure of Farkeria 

 while living, it may be now added that the fibre of which the 

 tissue is composed was probably homogeneous and solid, also 

 like that of Chitina ericojpsis and the recent chitinous species 

 of Hydractinia, but that during fossilization it became trans- 

 formed into homogeneous, colourless, transparent calcspar (fig. 

 14, a), coated with a layer of granular yellowish calcite (fig. 

 14, ?>), so as (again using a familiar allusion) to resemble 

 strings of sugar-candy, in which the string or thread would 

 represent the fibre, and the sugar-candy the granular incrus- 

 tation of calcite around it ; at least this is what is presented 

 by a transverse section of the calcified fossilized fibre, but not 

 so in the silicified state, as the mounted section of a siliceous 

 Parkeria at the Museum of the Royal School of Mines shows, 

 where the fibre has no coating whatever. Subsequently the 

 tissues thus fossilized became infiltrated with homogeneous 

 translucent calcspar, as if they had been soaked in so much wax; 

 and thus the whole st]-ucture also became entirely or partially 

 solidified, so as to assume the spherical form originally pos- 

 sessed by Parkeria, but in a lapidified state. Owing, how- 

 ever, to the infiltration being frequently partial, the central 

 portion often remains Mwinfiltrated, so that here the structui'e 

 is composed of the coated fossilized tissue-fibre only (fig. 13, d). 

 Such is the case with one of the specimens I possess, in which, 

 as before stated, this portion (fig. 13, d) about 5-l2ths inch 

 in diameter, is broken across so as to expose the centre, and 

 was originally contained in a shell or infiltrated zone about 

 5-24ths inch in thickness (fig. 13, a, h), so that, when entire, 

 the diameter of the whole specimen amounted to about 



