Professor Yakovlev — On Rugose Corals. 



Ill 



relation to the primary septa — a condition peculiar to the Rugosa. 

 The works of Duerden, Carruthers, and others have shown that the 

 secondary septa develop in four adjoining (and in four only out of 

 six) primary interseptal chambers, and are bilaterally symmetrical 

 in their arrangement. Two of the primary septa lie in the plane of 

 bilateral symmetry of the coral. About one of these — the Main- 

 septum — the secondary septa (as seen at their outcrops on the wall 



Text-fig. 2. — Diagram showing the arrangement of the outcrops of the septa 

 on the surface of the polyparium, according to whether the mouth is at 

 right angles to the concave side (above), to the convex side (below, on the 

 left), or to neither (below, on the right). The mouth is always at right 

 angles or nearly at right angles to the Counter-septum. S, Alar-septum. 

 act, a'a', the edges of the polyparium at different periods of its growth. 

 (After Yakovlev, 1904.) 



of the polyparium) are arranged pinnately ; while on each side of 

 the other — the Counter-septum — the secondary septa lie parallel 

 both to it and to each other (see Text-fig. 2). Two other primary 

 septa — the Alar-septa — lie somewhat at right angles to the Main- 

 and Counter-septa, thus dividing the calice into four quadrants, two 

 Main- and two Counter-quadrants. As seen at their outcrop on the 



