86 TABULATE CORALS. 



essentially polygonal. This is clearly shown by sections at 

 right angles to their course, and especially by those which 

 cut perpendicularly across their axial portions (PL IV., fig. 3 c). 

 The walls of the corallites, under all circumstances, remain quite 

 distinctly recognisable in thin sections, but the visceral chamber 

 becomes more and more contracted as we approach the mouths 

 of the tubes, by a dense secondary deposit of sclerenchyma (PI. 

 IV., figs. 3 <5, 3 c). As the result of this thickening of the 

 walls, the calices assume the form of rounded apertures encircled 

 by a prominent thickened margin (PI. IV., fig. 3 a), the true 

 wall being still discernible as a raised thread-like line describ- 

 ing a polygon round the central opening. In transverse or 

 tangential sections (PI. IV., figs. '^ b, -^ c), the visceral chamber 

 is seen to be reduced to a comparatively small circular or oval 

 tube; and in longitudinal sections (PI. IV., fig. 3 d) the same 

 feature is shown, while it is seen that the secondary deposit of 

 sclerenchyma is laid down irregularly, so as to constrict the 

 visceral chamber unequally at different points, but always most 

 markedly towards the mouth. Vertical sections also show that 

 the tubes are crossed by a few comparatively remote, complete, 

 and approximately horizontal tabulae, and that the walls are 

 pierced by a few larged-sized and irregularly placed mural 

 pores. In many instances, transverse or tangential sections 

 coincide in places with the plane of a mural pore, and then 

 the visceral chambers of contiguous corallites are shown as 

 laterally continuous (PL IV., fig. 3 d). Septa are sometimes 

 not recognisable at all ; but in other instances their presence 

 can be detected in the form of very rudimentary spiniform or 

 tubercular projections into the interior of the visceral chamber. 

 One of the nearest allies of the form here selected as the 

 type of Pachypora ccrvicornis, De Blainv., is the P. (Favosites) 

 crisiaia, E. and H., of the Upper Silurian. The latter, how- 

 ever, is distinguished by the constant presence of minute calices 

 interspersed among the larger annular apertures, by the smaller 

 size of average examples of the corallum, and by the possession 

 of more numerous tabulae, while the septa are often long and 



