1 5 o TABULATE CORALS. 



themselves as canals which run right across the walls of 

 two contiguous corallites, and place their visceral chambers 

 in direct communication. In vertical sections (fig. 23, B and D), 

 on the other hand, the mural pores may present themselves 

 under one or other of two forms, or under both of these. 

 Thus they may appear either as canals running directly across 

 the wall between two contiguous tubes, or (if the section hap- 

 pen to coincide more or less nearly with the plane of one of 

 the walls) as scattered circular pores situated within the space 

 bounded by the lateral walls of the corallites. All the Favosi- 

 tidcz may show the phenomena just described ; but in Pleuro- 

 dictyum and some other types thin sections show the existence 

 of another set of canals, which appear to run entirely within the 

 substance of the walls, and to have a course parallel with the 

 flat surfaces of the latter. The canals in question are best 

 seen in transverse sections (fig. 23, c), where they are shown 

 as distinctly circumscribed circular openings, placed at the 

 point of union of the walls of two contiguous corallites, or 

 situated within the actual substance of the wall. Similar open- 

 ings are seen in long sections (fig. 23, D), but the evidence is 

 here not so satisfactory as in the preceding case, since we 

 might be dealing with mural pores passing through the angles 

 of the prismatic corallites. What these canals are, it is difficult 

 to say, but they are certainly distinct from the " mural pores ; " 

 and as it is convenient to have a distinct name for them, I shall 

 call them the " intramural canals." Possibly they may only 

 be due to imperfect calcification or coalescence of the walls of 

 the corallites ; but that they are distinct from the mural pores 

 is shown by the fact of their occurrence in forms like Lyopora, 

 Nich. and Eth. jun., in which these latter openings are not 

 known to occur. In Pleurodictyum the " intramural canals," 

 as just defined, are neither numerous nor very conspicuous. 

 They are, however, present in a marked form in Columnopora, 

 Nich., and in Lyopora, Nich. and Eth. jun., in both of which 

 they appear to have been regarded as " coenenchymal tubules ; " 

 though their sparse and scattered arrangement, and their total 



