112 Professor Yahovlev — On Rugose Corals. 



wall of the polyparium, the secondary septa are pinnately arranged 

 with regard to the Alar-septa in the Counter-quadrants and lie 

 parallel to the Alar-septa in the Main-quadrants (see Text- fig. 2 

 above) . 



Such an arrangement of the secondary septa appears to be the 

 natural outcome of the mechanical conditions imposed by the curved 

 state of the conical polyparium. For the mouth of the conical 

 polyparium is generally in a plane perpendicular to its concave side 

 (Text-fig. 2 above) ; and the secondary septa have a tendency to 

 grow (as in all Authozoa) in a plane perpendicular to that of the 

 mouth of the calice. Since the primary septa are already existent 

 in a curved polyparium, the secondary septa can grow in a manner 

 just stated, and unimpeded, in two only of the four Primary 

 quadrants, namely in the two Counter-quadrants. In the two other 

 quadrants, namely the Main-quadrants, the secondary septa cannot 

 grow in a plane perpendicular to that of the mouth, because, at least 

 in part, they would be impeded by the convex surface of the Alar- 

 septa (see the dotted line t-t in Text-fig. 2 above). Therefore they 

 grow, as may be observed, so that their outcrops on the coral-wall 

 are parallel with those of the Alar-septa. 



But the Main-septum, sometimes, though comparatively rarely, lies 

 •on the concave side of the coral. When this is so, the plane of the 

 mouth of the calice is perpendicular to the convex side of the coral 

 (see Text-fig. 2, below and on the left). Applying to this case the 

 principle that the secondary septa tend to lie at right angles to the 

 plane of the mouth, we see that they can only thus grow unimpeded 

 in the quadrants lying on the convex side of the coral (now the 

 Counter-quadrants), whilst in the other pair of quadrants the growth 

 •of secondary septa perpendicular to the plane of the mouth would 

 be impeded by the concave face of the Alar-septa. 



A third, intermediary type also occurs (Text-fig. 2, below and 

 light-hand side) in which the plane of the mouth is inclined approxi- 

 mately equally to the convex and concave side of the polyparium. 

 In this case the Main-septum occurs on the convex side of the coral 

 as it did in the first instance. 



The fossilise arise as a necessary consequence of the primate 

 arrangement of the secondary septa. For there is always a com- 

 paratively broad space left between the youngest secondary septa and 

 the Main-septum in the two Main-quadrants, and between the 

 youngest secondary septa and the Alar-septa in the Counter- 

 quadrants (R, E x of Text-fig. 2 on the left) into which the soft 

 tissues settle down and contribute to the widening of these spaces, 

 converting them into permanent cavities. These spaces are the 

 Main- and the Alar-fossulse. The Main-fossula placed on the 

 convex or concave side of the corallum, according to the position of 

 the Main-septum, is really two juxtaposed fossulae, but appears as 

 one, because of the shortness of the Main-septum. Consequently it 

 is wider than the Alar-fossulse and is more constantly conspicuous 

 than these. The shortness of the Main-septum is caused by the 

 soft tissues settling down into the Main-fossula. This sagging of 

 the soft tissue, pressing also against the sides of the fossula, caused 



