Structure of Strom atopora. 255 



was occupied by the fleshy part of the animal, and thus was 

 the coenosarcal cavity of the ccenenchyma ; but, although the 

 horizontal lineation forming the triangular spaces was only 

 impressed upon the ccenosarc, it nevertheless alone formed 

 the upper and under lines of the rectangular spaces in the 

 vertical section (fig. 3, b b) • while such laminae being succes- 

 sively formed, at last produced the great corallum. That the 

 horizontal lineation was only impressed upon the ccenosarc is 

 evidenced by the decomposed fossil structure, in which the 

 parts occupied by the ccenosarc, i. e. the transparent, have 

 become opaque calcite, while all the rest have disappeared or 

 only left a brown stain. Thus the white calcite, bearing 

 merely the impressions of the horizontal lines, would remain 

 continuous throughout the lamina in other respects, but for 

 the intervention of the vertical lines or rods — finding its 

 openings only through the triangular spaces in the surface. 

 All this is further confirmed by the structure of the undecom- 

 posed lapidification, where the reverse of colour is the case, 

 and the horizontal lines and vertical rods are composed of 

 opaque white, while the intervening portion is filled with 

 transparent calcite, presenting, from its transparency, a dark 

 colour. When, from the undulating form of the layer or 

 other causes, the horizontal section is slightly oblique, the 

 pattern of the triangular lineation is not so complete, while 

 where it is entirely absent the ends of the rods of the vertical 

 structure alone come into view (PL XV. fig. 2, b), and vice 

 versa. 



The size of the triangular spaces varies slightly with 

 the structure of the ccenenchyma, which may be finer or 

 coarser according to the specimen or species, or from the 

 variable size of the polyps which had to protrude through 

 them ; but while they slightly vary among themselves, per- 

 haps the average diameter of the largest maybe from 1-1 80th 

 to 1 -120th inch. Those of Hydractinia pliocena* are only 

 l-360th inch, of H. calcarea l-600th inch, and of II. echinata 

 about l-266th inch in diameter, the smaller triangular 

 spaces being for younger polypites and other appendages that 

 might have existed in Stromtopora, as seen in the living 

 Hydractinia echinata } or when it has been preserved in spirit 

 with the polyps still exserted. 



The size of the rectangular spaces in the vertical section 



* Hydractinia pliocena, Allman, should, as Dr. G. Steinrnann has stated 

 (' Neues Jahrbuch f. Min., Geol. u. Paleontologie,' 1879, Heft v.-viii. 

 p. 733), be now Hydractinia incrustans, Goldfuss, from a gigantic speci- 

 men over 50 centime, in extent, at the Strasburg Museum, called by 

 Goldfuss " Stromatopora incrustans" (Bronn, Ind. Pal. p. 1203). 



18* 



