Corals of the Group liugosa. 115 



been published the interesting researches of Duerden *, I had, 

 in the first place, the opportunity of verifying the results of 

 Duerden's labours, which are of certain importance in estab- 

 lishing the general characteristics of the Rugosa, and, 

 secondly, of adding a few data to these characteristics. 



As is known, the distinguishing feature of the Rugosa is 

 considered to be the fact that they possess four primary septa, 

 of which two — the main septum and the counter septum — are 

 in the plane of symmetry of the coral, and the other two — 

 the alar septa — on either side of the plane. Besides, in 

 the quadrants between the primary septa the secondary septa 

 are arranged pinnately as regards the main septum in the 

 quadrants which adjoin it, and parallel with regard to the 

 counter septum in the counter quadrants. 



The septa belong to two cycles, of which one consists of 

 large and the other of small septa. It is interesting to 

 observe the way in which the septa are developed in the coral. 

 As proved by Duerden, the septa of one cycle — the small 

 ones — appear comparatively late, simultaneously, and at a 

 certain height. As to the septa of the other cycle — the prin- 

 cipal ones — their mode of development has led Duerden to 

 approximate the Rugosa to the group of now living Actinia — 

 Zoanthege, — their development precluding the possibility of 

 approximating them, as is generally done, to the Hexacoralla, 

 which form a skeleton, and of regarding the former as the 

 progenitors of the latter. 



The section (fig. A, p. 116) nearest to the pointed end of 

 the coral is 2'1 ram. in diameter, and represents twelve septa, 

 of which (according to Duerden) are to he regarded as primary 

 not four J as usually accepted for the Rugosa, but six septa, 

 which are marked in the figure by the cipher I, Four of them 

 are : the main septum I (H), the counter septum I (G), and 

 the alar septa I (S) — the two remaining septa I being situated 

 next to the counter septum, and forming with it interseptal 

 chambers in which (and exclusively in them) no new septa 

 of the same cycle are developed. 



Comparing figures A and B, we notice that the difference 

 between them is but slight, consisting chiefly in this, that the 

 main septum in the Russian specimen is situated on the convex 

 side of the coral, and in the American on the concave side. The 

 same coral in both specimens is bent in an opposite direction. 

 The observed relationship between the degree of development 

 of the main septum and the counter septum in a radial direction 



* J. E. Duerden, " On the Relationships of the Rugosa to the livinfi- 

 Zoanthe.T," Ann. & Mag. Nat. Hist., May 1902, p. 381. " 



8* 



