chief Generic Types of Paheozoic Corah. 301 



tiiiuing to grow uninterruptedly onwards. In some forms (as 

 L. duplicataj Mart.) the corallites always remain completely 

 free laterally (PL XVI. fig. 2) ; in others (as L. floriformis, 

 Flem.) they become united laterally, and assume a polygonal 

 form from mutual pressure (PI. XVI. fig. 3) ; whilst in L. 

 rugosa, M'Coy, there is an intermediate state of parts, and 

 the corallites, though usually free laterally, sometimes become 

 more or less amalgamated (PI. XVII. fig. 1). 



The corallites are always enclosed in a complete epitheca of 

 greater or less thickness, which exhibits fine encircling strige 

 and often Avell-marked accretion-ridges. 



The central area of the corallum is occupied by well-developed 

 tabulas, which are seen in longitudinal sections (PI. XVII. 

 fig. 1) to be extremely close-set. They often anastomose with 

 one another, and are very distinctly elevated as they approach 

 the columella. Hence in transverse sections (PI. XVI. figs. 1 A 

 & 2 a) the divided edges of a greater or less number of the 

 tabular may be seen surrounding the columella. 



The tabulee are pierced centrally by the columella, which 

 runs continuously from the bottom of the visceral chamber to 

 the floor of the calice, where it appears as an elevated acutely 

 conical prominence. The columella is of large size, approxi- 

 mately cylindrical in shape, and composed of numerous 

 twisted plates, which appear to become continuous laterally 

 with the tabular, as shown by longitudinal sections (PI. XVII. 

 fig. 1) . In transverse sections (PI. XVI. figs. 1-3) the colu- 

 mella is seen as a conspicuous central cellular mass, the outer 

 portion of which is formed by concentric lines disposed in 

 successive sectors. 



The space between the central tabulate area and the inner 

 surface of the wall is occupied by vesicular tissue. The 

 vesicles of this zone are of comparatively very large size ; and 

 they are formed by strongly arched plates, the convexities of 

 which are turned upwards, and which are seen in longitudinal 

 sections (PI. XVII. fig. 1) to have a direction outwards and 

 upwards. In transverse sections (PI. XVI. figs. 1-3) the 

 divided edges of the vesicles of this zone form an exceed- 

 ingly conspicuous feature, partly from their large size and 

 partly because they are unencumbered by the septa. A 

 distinct accessory wall is sometimes clearly present, inter- 

 vening between the central tabulate area and the outer vesi- 

 cular zone. In other cases the apparent inner mural invest- 

 ment seems to be little more than an appearance produced 

 by the contrast of structure between the central tabulate area 

 and the peripheral zone of vesicular tissue at their line of 

 junction. 



