STROMATOPOROIDEA. 233 



are formed by radiating processes given out in whorls at definite 

 levels from the radial pillars, and uniting with one another in such 

 a way as to give rise to an angular meshwork. Hence, horizontal 

 sections of the species of this genus (fig. 1 1 5, a and c) exhibit 

 a structure not at all unlike that of a Hexactinellid Sponge, the 

 cut ends of the radial pillars constituting so many centres from 

 which the horizontal processes forming the laminae are given out 

 in a stellate manner. The earliest types of Actinostroma appear 

 in the Silurian rocks, but the genus is characteristically Devonian. 

 In the nearly allied genus Clathrodictyon, the radial pillars are com- 

 paratively ill developed, and do not pass from one interlaminar space 

 to another. The species of this genus, though not absolutely unrep- 

 resented in the Devonian, are characteristic of the Silurian period. 



The singular genus Labechia is the type of another group of 

 Stromatoporoids. The ccenosteum in this genus (fig. 117) is usually 

 laminar, attached by a peduncle, and 

 having the under surface covered with 

 a wrinkled calcareous membrane or 

 " epitheca." The upper surface shows 

 no traces of pores or zooidal apertures, 

 but is covered with numerous blunt 

 tubercles. Vertical sections (fig. 116, b) 

 show that these tubercles are the free 

 upper ends of stout radial pillars, which ^'S^TiSSta 

 are connected with one another by rocks of Gotland, of the natural size, 



J showing the epithecate under side 

 nUllierOUS arched CalcareOUS plates. and the tuberculate upper surface. 



Horizontal sections (fig. 116, a) show <° ri s inal > 

 the cut ends of the radial pillars, united by the calcareous plates 

 above spoken of. The pillars are hollow, with a minute axial 

 canal, but they do not seem to be naturally perforated at their 

 summits. In the absence of definite zooidal pores, it must be 

 supposed that the surface was covered with the ccenosarc, from 

 which the polypites were budded off. The genus Labechia ranges 

 from the Ordovician to the Devonian, and the allied Rosenelia is 

 found in the Ordovician and Silurian rocks. 



In this connection a few words may be here said with regard to 

 the extraordinary and problematical fossils from the Ordovician rocks 

 of North America for which Mr Billings founded the genus Beatricea. 

 The fossils in question have the form of cylindrical or angular stems 

 (fig. 118), which are nearly straight, are unbranched, and may attain 

 a length of several feet. The outer surface sometimes shows small 

 rounded tubercles, or may exhibit minute circular apertures, the 

 nature of which is quite uncertain. Transparent transverse and 

 vertical sections (fig. 1 1 8, b and c) show that the fossil is prin- 

 cipally made up of a thick outer sheath of lenticular calcareous cells 



