﻿LABECHIA CONFERTA. 159 



In internal structure the ccenosteum consists of stout, circular or oval, radial 

 pillars, which have a diameter of i to ^ mm., and terminate upwards in pointed 

 extremities, each being traversed by a central canal. The pillars give rise to 

 radiating " arms " or plates, which unite with one another in such a manner that 

 the entire space between the pillars becomes filled with a tissue of calcareous 

 vesicles, the convexities of which are directed upwards. 



Obs. — This well-known species occurs typically in the form of laminar ex- 

 pansions, with an epithecate base and peduncle of attachment ; but in some 

 instances an encrusting habit of growth is observable. Young examples (Plate 

 III, figs. 9 and 10) may be only 2 or 3 cm. in diameter and 1 mm. in thickness ; 

 but old specimens may be of greater size, perhaps a foot in diameter, and may 

 reach a thickness of 2 — 3 cm. A single specimen often consists of two or more 

 superposed colonies. The surface differs from that of many Stromatoporoids in 

 the complete absence of " mamelons " and of any indications of an astrorhizal 

 system, though studded throughout with prominent tubercles, which may be about 

 \ mm. in height, and about the same diameter at their base. The tubercles 

 may be placed about | to j mm. apart, or may be in contact, often coalescing 

 in sinuous rows. The apices of the tubercles may be simply rounded or pointed, 

 and may be apparently imperforate. In other cases a distinct circular aperture 

 may be detected at the apex of a pillar, though it is not clear that this is not the 

 result of weathering. 



Vertical sections (Plate XX, fig. 1 ; and Fig. 18, B) show that the coenosteum 



Fig. 18. 



i fy ^\l 



r 



Fig. 18. — Sections of Labechia conferta, Lonsd. sp., enlarged twelve times. Wenlock Limestone, Iron- 

 bridge. A. Tangential section. B. Vertical section, pp. Radial pillars, c c. Connecting-processes 

 or " arms." 



is essentially composed of very stout radial pillars which spring from the basal 



epitheca and are continued to the upper surface, where they terminate in the 



prominent tubercles above spoken of. The interspaces between the pillars are 



occupied by a vesicular tissue formed by the coalescence of connecting-processes 



or " arms," given out from the pillars, the convexities of the vesicles being turned 



towards the upper surface. 



22 



