116 



On Corals of the Group Rugosa. 



is in all probability owing to the direction of the bend; the 

 primary septum on the concave side (in the Russian specimens 

 the counter septum and in the American specimens the main 

 septum) is short and the primary septum on the convex side is 

 long (figs. A and B). It seems to me that, with regard both to 



1/5) 



16) 



A and B, the corresponding sections of the Russian and American 

 specimens (the latter after Duerden, modified as regards the main 

 septum and the counter septum, see below, at end) of L'oj>Jiu- 

 pliyllum pi oliferum : the pi irtions of the sections turned upwards 

 lie on the convex side of the coral. The primary interseptal 

 chambers in which no new septa are formed are striated. I, 

 I (II), I (G), I (S), the primary septa; 1, 2, the later principal 

 septa. 



the main septum and the counter septum, the fact may be 

 easily explained by supposing that the bend of the coral on the 

 concave side causes a contraction, affording less space for the 

 development of the septa than on the convex side ; the former 

 is characterized by contraction, the latter by distention. 



This assumption is strengthened by another peculiarity of 

 the coral, viz. that of the four primary interseptal chambers, 

 in which the successive principal septa are generally deve- 

 loped, the two situated nearest to the convex side develop the 

 septa more rapidly (in greater number). These chambers 

 are not the same in the Russian and American specimens : in 

 the former (fig. A) they are contiguous with the main septum, 

 in the latter they are separated from it as well as from the 

 counter septum by other primary chambers. 



In examining the two specimens (tigs. A and B) we must 

 also notice that in two of the four primary chambers no new 

 septa are formed — invar iably in those primary chambers which 



