MADREPORARIA RUGOSA. 28 1 



Thus in PalcEocyclus porpita (fig. 158), the number of septa in each 

 quadrant is sometimes the same — namely, nine — making, with the four 

 principal septa, a total of forty. In most examples of this coral there 

 are, however, forty-four septa in all, the two counter-quadrants con- 

 taining each eleven septa, while the cardinal quadrants have nine 

 each. 



The peculiar arrangement of the septa above indicated can be 

 recognised in a large number of the simple Rugose corals by a 

 mere inspection of the surface of well-preserved examples. Where 

 the surface is badly preserved, and the observer has to rely upon 

 thin sections, the "cardinal" and "counter" septa can usually 

 be readily recognised, but it is often a matter of difficulty to deter- 

 mine the "alar" septa. In some of the most completely radial 

 forms — as in some species of Cyathophylhim — the position of the 

 four principal septa may cease to be recognisable in transverse 

 sections of the corallum. Lastly, in cases where the septa are rudi- 

 mentary — as in the Cystiphylloids generally — it may be impossible 

 to demonstrate the tetrameral arrangement of the septa, though in 

 other cases (as in Goniophylluni) no such difficulty obtains. 



As regards the relative length of the septa, the most usual arrange- 

 ment is to find that the septa are alternately equal (approximately 

 equal, that is to say) ; so that the septa consist of a long series and 

 a short series alternating with one another. Hence, the septa are 

 often said to be of two " orders," but the septa of each order are 

 by no means always quite equal in length, nor is it to be supposed 

 that the septa of each order were simultaneously developed. 



In a great many of the Rugosa, the bilaterality of the corallum is 

 especially marked by the unequal development, as regards length, of the 

 " cardinal" and " counter" septum, one or other of which usually occupies 

 the so-called " fossula." The "fossula" or "fossette" (fig. 139), as pre- 

 viously explained (see p. 262), is a more or less conspicuous groove or 

 depression in the calice, due to a reduction in size of the septa at that 

 point, necessitated, in all probability, by the existence in the living polype 

 of % one or more hypertrophied mesenteries carrying the reproductive 

 organs. As a rule, there is only one " fossula," which may be median or 

 lateral in position, the latter being, however, very unusual. Sometimes 

 there are three fossulae, one median, and two lateral ; while in other cases 

 {e.g., Omphyjnd) there are four of these calicine grooves, two median, and 

 two lateral. In this last case (fig. 159, a), the four fundamental septa 

 occupy the four fossulse, the cardinal septum being the best developed. 

 In the much more common case of there being only a single median 

 fossula, this is almost always related to either the " cardinal " or the 

 " counter " septum. The most usual arrangement is for the fossula to be 

 placed on the convex (or " dorsal ") side of the simple coralla, in which 

 case it is intersected mesially by the " cardinal septum," which is then 

 much reduced i?i size. This disposition of parts occurs in Paltzocychis 

 porpita (fig. 158), in the genus Strepteiasma, and in most of the species 

 usually referred to Zaphrentis. Sometimes, as in Lophophyllum, the 

 " cardinal " septum is very short, and occupies the dorsally-placed fossula, 



