J. E. DUERDEN. 



a. 



single corallite. 1 It is seen that each alar or lateral region is that 

 at which new septa (a-e) are successively added to the primary six. 

 Furthermore, the additions take place in such a manner that the 

 newer, shorter septa are for some time inclined towards the older, 

 and are fused with them in a unipinnate manner by their inner 

 borders; it is only towards the close of development (Figs 10, 

 1 1) that all the septa become free, and are then radially arranged. 

 This inclination and fusion of the newer with the older septa, as 

 well as their smaller size, gives a distinctive character to the alar 



regions in Streptelasma during 

 the various developmental stages, 

 and usually results in a special 

 interval or inter septal space on 

 the dorsal side of each alar 

 septum. 



If now the alar regions of Figs. 

 2 10 be compared with those of 

 Fig. i, the two alar fossulae in 

 Hadrophyllum are seen to cor- 

 respond with the two lateral 

 regions of addition of new septa 

 in Streptelastna, and both are 

 within the two middle of the six 

 primary interseptal chambers. 

 Hence each alar fossula in the 

 adult corallite of Hadrophyllum 

 really corresponds with ontogenetic stages in the establishment 

 of the septa in Streptelasma, the septa concerned being those 

 immediately dorsal to the primary alar septum. 



That no alar fossula is present at the more mature stages of 

 Streptelasma (Figs. 9-11) is due to the fact that as the coral at- 

 tains its full development the septa become free from one another 

 at their inner border, and at the same time become equal in size 



1 The septal sequence here illustrated is in accordance with Kunth's well known 

 law of septal development found to be characteristic of the Rugosa. Hitherto, it 

 has been generally assumed that only four primary septa are piesent in tetracorallids 

 whereas six actually occur. However Haeckel's term Tetracoralla has still an ap- 

 propriateness since the subsequent septa are formed within only four of the six pri- 

 mary interseptal chambers in contrast with the six in Hexacoralla. 



FIG. 3. A bilateral pair of meta- 

 septa (a) has appeared, a member 

 within each of the middle chambers or 

 counter quadrants, inclined towards its 

 corresponding dorso- lateral primary 

 septum. 



