Professor YaJcovlev — On Rugose Corals. 



113 



the deviation of the free edge of the secondary septa towards the one 

 preceding it, so as, finally, to fuse with it (a with S l and /, and 

 b with a, in Text-fig. 3), giving the appearance (it is, of course, only 

 an appearance) of repeated branching of the primary septa. The 

 septal fusion occurs in young individuals as well as in mature ones. 

 The older parts of many polyparia become cylindrical and straight 

 instead of conical and curved. In these there are no wide spaces 

 between the primary and secondary septa, and consequently there is 

 less development of the f ossulse, and the fusion of the septa disappears — 

 in a word, radial symmetry, doubtless existent in the progenitors of 

 the Rugosa, is re-established. 



Since the fourth fossula, that lying on each side of the Counter- 

 septum, is rarely visible in the Rugose skeleton, its origin must be 

 different from that of the other fossulse. Its presence is due to the 

 fact that there are no secondary septa adjacent to the Counter- 

 septum. A study of the functions of the other fossulae (of course 



kg) r 



Text-fig. 3. — Diagram of the calice of Hadrophyllum pauciradiatum, 

 Edwards & Haime. H, G, S, Main, Counter, and Alar septa respectively. 

 a-e, secondary septa. F, F, F, Fossulas. (After Duerden, 1905.) Nat. 

 size. 



a purely hypothetical consideration, dealing, as it does, with the soft 

 parts of an extinct group of animals) may help to explain the origin 

 of the Counter-fossula. It appears to me that the function of the 

 Main- and Alar-fossulse was to bring water to the axial part of the 

 coral. This is partly corroborated by the fact that all three lie in 

 one general direction, since the Alar-septa make an acute angle with 

 the Main-septum. I have established the hypothesis that the 

 solitary, curved polyparia of the Rugosa, during their life, were 

 orientated on the sea-bottom with their convex sides towards the 

 prevailing currents, a position most advantageous from a mechanical 

 point of view (see Text-fig. 1). This orientation itself would cause 

 the water to flow along the Main- and Alar- fossula? towards the axial 

 parts of the coral; and the water would naturally find its exit on 

 the opposite side, namely along the Counter-fossula, which doubtless 

 arose for this purpose. For, though not constantly expressed in the 



DECADE VI. — VOL. IV. — NO. III. 8 



