114 



Professor Yakovlev — On Rugose Corals. 



skeleton, the Counter-fossula would then be too important not to be 

 constantly expressed in the soft parts of the coral. If this were so, 

 the presence of the Counter-fossula in the soft tissues might actually 

 impede the formation of secondary septa in the loculi adjacent to the 

 Counter-septum (Text-fig. 4). 



A last peculiarity of the solitary Rugosa correlated with the 

 curved, conical polyparium, is the fact that the number of secondary 

 septa present in the quadrants on the convex side, is sometimes 

 greater than the number of secondary septa on the concave side 

 (Text-fig. 4). Consequently, the position of the axis of the coral, as 

 denned by the point of meeting of the primary septa within the 

 polyparium, appears displaced towards the polyparium's concave 

 side. This phenomenon is no doubt due to retardation of development 

 on the concave side, as though under the influence of contraction of 

 the coral here at its curved part ; on the convex side, on the other 



Text-fig. 4. — Diagrammatic sections of Lojphophyllumproliferum, McChesney. 

 The section on the left is a Eussian and on the right an American 

 specimen. In both cases the upper side of the section is on the convex 

 side of the polyparium (after Yakovlev & Duerden, 1903). H, G, S, 

 Main, Counter, and Alar septa respectively. Si, the fourth pair of primary 

 septa, a, b, secondary septa. § nat. size. 



hand, the development is accelerated, as though under the influence 

 of distention. 



Colonial Rugosa are characterized by intracalicinal budding, 

 resulting in a bush-like appearance, and causing a limited growth of 

 the colony. Hexacorals, on the other hand, multiply by division and 

 build colonies which spread far from their initial point on the 

 surface of the substratum. Colonial Rugosa also differ from Hexa- 

 corals in the greater size of their individual corallites. The Rugosa, 

 consequently, do not possess the plasticity necessary for producing 

 the variety of colonial forms that we find among the Hexacorals, 

 e.g. Madrepora. To the small dimensions of the colonies and to the 

 monotony of form probably was due the small share they took in 

 building Palaeozoic reefs. 



The Rugosa became extinct at the close of Palaeozoic time, probably 

 in consequence of an unfavourable environment, which may have 



