MADREPORARIA RUGOSA. 



-'." 



coral, such as Streptelasma corniadum or Ompkyma sitbhirbinata. 

 From an investigation of such types it was shown by Kunth that 

 the symmetry of the corallum is tetrameral, and is governed by four 

 principal septa placed at right angles to one another. The bilater- 

 ality of the corallum depends upon the mode in which new septa are 

 developed in relation to three out of these four principal septa. The 

 most important of the three principal septa in question is placed 

 along the convex (or " dorsal '') side of the corallum, and new septa 

 are developed on both sides of this in a pinnate or feather-like 

 manner (fig. 156, a). This dorsal median septum may be longer or 

 shorter than the other septa, or of normal size, and it may be con- 



Fig. T56.— a, Streptdasma corniadum, viewed from its convex (or "dorsal - ') side, she 

 the intercalation of new septa on both sides of the "cardinal septum" (A), of the natural size. 

 b, Side-view of the same specimen, showing the development of new septa on one side of one of the 

 "'alar 7 ' septa (s). From the Ordovician rocks of North America. (Original.) 



veniently spoken of as the " cardinal septum " (the " Hauptseptum " 

 of Kunth). The septa produced on the two sides of the " cardinal 

 septum " gradually get less and less oblique to the latter, till we reach 

 two septa placed laterally, at right angles to the " cardinal septum,''" 

 which may be spoken of as the "alar' 7 septa (the " Seitensepta " of 

 Kunth). On the ventral side of each alar septum — i.e., on the side 

 furthest removed from the " cardinal septum " — new septa are devel- 

 oped in an oblique manner (fig. 156, b, s). The remaining septum 

 of the quartette of principal septa is placed in the middle line on the 

 concave (or "ventral") side of the corallum, and it does not serve 

 as a starting-point for the development of new septa, and therefore 



