STROMATOPORA CONCENTRICA. 165 



XXI, fig. 1). Bach latilamina is made up of a number of trabecular concentric 

 lamina?, which are simply curved or are usually but slightly undulated, the surface 

 being, therefore, usually free from prominences. Pointed eminences or " mame- 

 lons " are, however, present in one variety of the species (8. concentrica, var. 

 colliculata, Nich.). 



Astrorhiza? are usually fairly numerous and are generally of small size (Plate 

 XXI, fig. 3). In one form (var. colliculata) the astrorhiza? are disposed in vertical 

 systems, and are often surrounded by concentrically disposed lamina?, thus giving 

 rise to " astrorhizal cylinders;" and in another form, which may be provisionally 

 referred here (var. astrigera, Nich.), the astrorhiza? are large and spreading. The 

 surface of the coenosteum, when well-preserved, exhibits vermiculate and inoscu- 

 lating ridges corresponding with the reticulated skeleton (Plate XXI, fig. 3). 



As regards its internal structure, the coenosteum is of the strictly " reticulate " 

 type, the radial pillars and horizontal connecting-processes being fused with one 

 another to form a continuous and complex network traversed by correspondingly 

 complex anastomosing canals (Plate XI, figs. 16 — 18). Distinct zooidal tubes, of 

 a somewhat irregular and tortuous form, are developed, and are crossed by a 

 moderate number of transverse partitions or " tabula?." The skeleton-fibre itself 

 is minutely porous in structure, and is from | to ^ mm. in diameter, the reticu- 

 lated tissue which it forms being thus very dense. 



Obs. — The typical examples of Stromatopora concentrica, Goldf., are usually 

 spheroidal or subcylindrical in shape, and vary from about an inch up to over a 

 foot in diameter. The latilamina? are concentric with the general surface, and are 

 either simply curved or are thrown into wide undulations (Plate XX, fig. 10). In 

 the form which I have named, 8. concentrica, var. colliculata, the coenosteum is 

 cylindrical or cylindro-conical in shape, and the latilamina? are rolled concentrically 

 round an imaginary central axis. Though the latilamina? form such a conspicuous 

 feature in this species (Plate XXI, fig. 1), it can hardly be said that this feature is 

 developed to a greater extent in 8. concentrica, Goldf., than it is in 8. Carteri, Nich., 

 or jS. tyjpica, Rosen, or in some other Stromatoporoids of entirely different affinities 

 (as, for example, in certain forms of Actinostroma stellulatum, Nich.). 



As regards its internal structure, 8. concentrica presents the completely reticu- 

 late skeleton of all the species of Stromatopora proper ; but the blending of the 

 radial and horizontal elements of the coenosteum is not so complete as to prevent 

 the ready recognition of the radial pillars in properly prepared vertical sections. 

 Owing to the comparative distinctness of the radial pillars (Plate XI, fig. 18) the 

 zooidal tubes are clearly marked out as irregular, often sinuous, vertical tubes, the 

 internal cavities of which are crossed by remote transverse " tabula?." Tangential 

 sections (Plate XI, figs. 16, 17) are, in general, readily distinguished from corre- 

 sponding sections of allied species of the genus by the comparative density and 



