280 



ZOANTHARIA. 



does not show any feather-like disposition of the septa on its sides. 

 It may be longer or shorter than the others, and may be spoken of 

 as the "counter" septum (the " Gegenseptum " of Kunth). 



Hence, the surface of the theca of a simple Rugose coral such as 

 Streptelasma corniculum is marked out by these four principal septa 

 into as many quadrants, as shown in the annexed sketch (fig. 157) 

 of a specimen of this coral, viewed from the base, and therefore 

 greatly foreshortened. Thus the dorsal or convex side is divided 

 into two " cardinal " quadrants, occupied by the pinnately developed 

 septa which flank the "cardinal septum" (fig. 157, h). The two 



Fig. 157. — Plan of the septa in a speci- 

 men of Streptelasma corniculum, viewed 

 from below, the radiating lines indicating 

 the peripheral margins of the septa as ap- 

 pearing on the surface of the corallum. 

 h, The "cardinal septum"; ss, The 

 " alar" septa ; g, The "counter septum." 

 (After Kunth.) 



Fig. 158. — Transverse section of 

 Palceocyclus porpita, from the Silurian 

 of Gotland, enlarged three times. 

 ^" Cardinal " septum, contained in 

 the fossula ; ss, "Alar" septa; g, ' 

 " Counter" septum. In the section 

 figured there are only forty septa, but 

 there are usually forty-four, twenty- 

 two long and twenty - two short. 

 (Original.) 



"counter" quadrants, on the other hand, are occupied by the septa 

 developed along the ventral side of the "alar" septa (fig. 157, ss), 

 and they merge with one another in the mesial ventral line, owing 

 to the fact that the " counter septum " (g) is not a source for the 

 development of new septa. It follows from the above that an inspec- 

 tion of the surface of a well-preserved example of Streptelasma cor- 

 niculum or Omphyma subturbinata enables us to recognise by the 

 disposition of the septa the position of the " cardinal " and " alar " 

 septa, but that the " counter septum " cannot be thus recognised. 



It was further pointed out by Kunth that though the four principal 

 septa above spoken of divide the corallum into four quadrants, it does 

 not necessarily follow that these contain the same number of septa 

 each. On the contrary, it is not unusual to find that the " counter 

 quadrants " contain conjointly a greater number of septa than do the 

 two "cardinal quadrants." 



