140 



TABULATE CORALS. 



iWM^^^^^ 





cal, or in the form of a flattened expansion, the under surface 

 of which is covered with a wrinkled epitheca. The corallum is 

 attached to some foreign body by the centre of its base, and 



in some instances 



.^.soi^^ll^'^T^s^ ^^^ epitheca gives 



off root - like pro- 

 longations in addi- 

 tion. In all the typi- 

 cal forms the coral- 

 lites are polygonal, 

 of larger size than 

 in the ordinary spe- 

 cies of Favosites, 

 and in close contact 

 throughout their en- 

 Fig. 21. — Michdinia coiwexa, D'Orb., viewed from above, tire length; and they 

 of tlie natural size, from the Comiferous Limestone of , . r , 



Ontario. (After Billings.) radiate from the Cen- 



tre of the base, those 

 occupying the circumference of the mass being so highly in- 

 clined as to be nearly parallel with the epitheca, while those 

 in the middle are more or less nearly vertical. The visceral 

 chamber, like the corallite itself, is usually polygonal ; but in 

 some cases {e.g., M. cylindrica, E. and H.), the tube-cavity 

 is constricted at intervals by ring-like ridges, which give it 

 a cylindrical appearance. The walls, though moderately stout, 

 have none of the secondary thickening of the interior so 

 characteristic oi Pachypora, Lindst., and its allies. The mu- 

 ral pores are numerous, circular, of variable size, and quite 

 irregular in their distribution. The chief noticeable feature 

 about the tabulae is their marked curvature, their convexities 

 being directed upwards. Moreover, the tabulse seldom extend 

 quite across the visceral tube, but in approaching the margin, 

 they usually unite with other smaller tabulae, so as to give rise 

 to the formation of numerous large-sized lenticular vesicles, 

 which are most abundantly developed near the sides of the 

 tubes. The extent, also, to which the tabulae thus become 



