232 Prof. J. E. Duerden on the 



the possibility of firmly establishing the relationships of the 

 group. The ingrained idea of a primary tetrameral plan for 

 tiie rugose corals rests upon the fact that in the mature 

 corallite there are frequently four ])rimary se|)ta, which by 

 their greater or less size stand out more or less conspicuously 

 among the rest and divide the calice into quadrants; and, in 

 addition, it is easily seen that new septa are added at four 

 regions, one within each quadrant. As demonstrated in my 

 two earlier papers, these suggestions of tetramerisra in both 



FiiT. 7. 



A further septum {(X) occurs witliin each middle chamber, witliout a 

 correspouding pair bein<r added Avithin the ventro-lateral chambers. 

 The central part of the dorsal directive septum is beginuing to be 

 freed from the inner ends of the other septa, and appears something 

 like a columella, with distinct centres of calcification. In the space 

 between any two adjacent principal septa the first traces of the 

 centres of calcification of the exosepta (.r) are nuw displayed, their 

 appearance all round the calice being nearly simultaneous. Tlie 

 exosepta have thus no ordinal value, such as the principal septa or 

 entosepta possess ; they con-espond with alternate external grooves 

 alternating with the principal septa. 



the developing and mature corallite are not at all proofs of a 

 primary tetramerism \ moreover, Gordon jiroduces no proofs 

 beyond a suggestive condition in Streptelasma profundum to 

 be discussed later. According to my interpretation, in the 

 Rugosa a secondary tttramerism has been impressed upon a 

 p rim a ri/ h exa m e r is m . 



At first sight it would seem to be a simple matter to 

 determine the number of primary septa in a corallum. It is 



