The Relationship of the Tetracoralla to the Hcxacoralla. 171 



the contour of the wall ; this lies just inside the angle formed 

 by the junction of the wall with the floor of the calyx, and 

 slightly below the level of the floor. Thus the base of each wall 

 is traversed by two such tubes bordering the margins of two 

 contiguous corallites. From these ring-canals, branches are given 

 off, which open by pores into the floor of the calices near the 

 base of the walls. Other branches are given off in the opposite 

 direction from the ring-canals and traverse the wall horizontally, 

 connecting the ring-canals of two adjacent corallites. Other 

 pores are occasionally seen, opening higher up on the walls of 

 the calices ; these are, however, more irregular in their distri- 

 bution. The pores opening round the basal margin of the calices 

 are fairly numerous, and are placed close together, the distance 

 between them being generally not much greater than the diameter 

 of the pores themselves. 



"In microscopic sections the walls and floors of the corallites 

 exhibit a finely crystalline fibrous structure, similar to that which 

 characterizes many recent corals. The long axes of the fibres 

 are arranged perpendicularly to the walls and floors of the calices. 

 There is no trace of the trabecular structure figured by Nichol- 

 son in his descriptions of Cleistopora and Palcuacis, while tabulae 

 are entirely wanting. This compact fibrous coenenchyma is per- 

 forated by the tubes described above; and, in the neighborhood 

 of the tubes, the fibres are arranged in a radial manner perpen- 

 dicularly to the walls of the tubes. Vertical sections cut at 

 right-angles to a corallite-wall show two perforations below the 

 base of tlie wall and on each side of it, representing transverse 

 sections of the two ring-canals of contiguous corallites. From 

 these, in many sections, tubes can be observed passing obliquely 

 outwards and upwards, and penetrating the floors of the calices 

 at the base of the walls, where they terminate at the surface to 

 form the pores already described. In horizontal sections, pre- 

 pared so as to expose the base of the walls a short distance 

 below the floors of the calices, the system of ring-canals and 

 their connecting tubes can be well seen, the canals being rendered 

 conspicuous by their infilling of darker argillaceous material." 



The author of the genus directs attention to resemblances to 

 Leptopora {Cleistopora) geometrica, and to Palceacis, Pleuro- 

 dictyum and MicrocyatJms. The following points of dift'erence 

 from Leptopora {Cleistopora) are cited: (i) The absence of the 



