238 PROF. H. A. NICHOLSON AND DE. .T. MURIE ON THE 



bling interlaminar spaces, and, moreover, the fossilized calca- 

 reous nature of tlie whole — uudoubtedly simulated in a forcible 

 manner the characters of Stro^natopora in vertical section. This 

 will readily be allowed if fig. 13, PI. IV., be compared with fig. 3, 

 PI. I. If, again, the surface of one of the layers of Eschara 

 nolilis, fig. 12, PL IV., be compared with the surface of the 

 concentric laminaj of Stromatopora tiiberculata, fig. 2, PI. I., less 

 likeness is discernible, though the weathered protruding pillars 

 and pseudo-orifices of the latter may at first sight be taken for 

 the cell-orifices of. the former. 



Closer inspection, however, does not bear out such apparent 

 agreement ; for the regularity in shape and position of the cells and 

 .orifices of the polyzoarium of this species of Eschara {TIemeschara, 

 Busk), avicularia, and other secondary features are completely at 

 variance with the structural surface-peculiarities of 8. tuberculata 

 and all other sj^ecies of the genus. But even the general similarity 

 of f acies of vertical sections between the two forms in question 

 diminishes in proportion as detailed and microscopic investiga- 

 tion is pursued ; and from such Stromatoporoid genera as Cauno- 

 pora, Stylodictyon, &c., the polyzoarium of Eschara departs still 

 further. Another genus of the family of Escharidae, namely 

 Metepora, occasionally, to a certain extent, simulates the minute 

 vesicular structure of Glatlirodictyon ; but the general fenestrated 

 foliaceous nature of its polyzoarium, presence of avicularia, mar- 

 ginal spines, ovicells, &c., suificiently distinguish Polyzooid from 

 Stromatoporoid organization. Among the family of Celleporidse 

 again, in certain of its forms, sufiicient likeness can be traced 

 to justify a comparison with Glatlirodictyon, and jjartially, it may 

 be, with Stylodictyo7i. Here, in the genus Gellepora, the erect di- 

 chotomously branched species must be excluded, and only the 

 globose spreading adnate forms taken into account. Even these 

 latter seldom, if ever, assimie the outward form of the Stromato- 

 poroids, not excepting the recent Gellepora onamviillata, with its 

 incrusting polyzoarium and surface-projections. Their vertical 

 or horizontal section in mass, though, does assume something of 

 the cellulo-vesicular character of similar sections of Glatliro- 

 dictyon. The heaping together and vertical inclination of the 

 cells of the polyzoarium of Gellepora is, however, only a deceptive 

 likeness ; for even in the fossil species of the genus the punc- 

 tured or sculptured character of the cell- walls, the presence of 

 rostra, avicularia, ovicells, and often denticles, sinuses, or spines, 



