Ilydiactinia, Parkeria, and Stromatopora. 57 



number and slightly in size outw^irds ; so that while they 

 average transversely about 1 -.'}()( )th inch in diameter near the 

 centre, their cavity is about l-12r)th inch in transverse dia- 

 meter at the circumference of a 8j)ecimen of Parkeria H inch 

 in thickness. On progressing outwards, these cylindrical 

 pillars, for the most part, lose their individuality from the 

 increase in quantity ot the tissue-fibre, which involves those 

 in its course as the latter assumes a columnar disjxjsition, 

 increasing in size outwards. The columns so produced thus 

 radiate from the centre to the circumference, and, arching 

 towards each other in all directions as they arrive at each 

 lamina, appear to divide the " interval," in the vertical section, 

 into a number of chambers. These are the " cliamberlets " of 

 Dr. Carpenter. 



So long as the vertical tubes retain their individuality — that 

 is, in tiic first three or four intervals, where they are not ob- 

 scured by the additional growth of the tissue-fibre (fig. 17, h) — 

 they, with the laminae of Parkeria, closely resemble the lamina?, 

 intervals, and tubes respectively of Tuhijwra mii^ica, especially 

 as the whole structure of the latter is elaborated out of a 

 similar tissue; but besides being almost incomparably larger 

 (that is, while the lamina, intervals, and tubes in Parkeria 

 are at the })art mentioned res})ectively l-900tli, l-2()0tli, and 

 l-90()th inch across, those of 7'. mnsica are l-24th, l-4tli, and 

 1-lOth inch across, the cross diameter of the interval indicating 

 the length of the tube in each instance), the tissue of T. musica is 

 not reticulated but sieve-like and laminiform, all the holes being 

 on the same plane and of all kinds of sizes, precisely like the 

 structure of the calcareous tissues in the Echinodcrmata. 

 The radiating tubes of T. musica, too, are for the most part 

 0])j)Osite each other, so as to appear vertically continuous for 

 a long distance, although internally their cavity is frequently 

 intcrruj)ted by a diaphragm of the same sieve-like tissue, 

 which is for the most ])art just below the lamina; and it is 

 worth noticing that while the openings in a vertical section 

 of the lamina of T. viusica resemble those in the lamina of 

 Parkeria, they are also present in a ring-like form inside the 

 tube of T. musica opposite the lamina — that is, just above the 

 diaphragm ; so that the radial tubes, as in Parkeria, were in 

 communication with the ])assages in the centre of the lamina, 

 and not so continuously hollow as at first sight they would 

 ap])ear to be. 



Having now described Parkeria from the vertical section, 

 let us tuni our attention to the surfaces of the lamina (that is, 

 the outer and inner surfaces), concent rically — an examination 

 which the same ?/niufiltratcd specimen renders comparatively 



