MORPHOLOGY OF THE MADKEPORARIA. 51 



maturity. They have been described as fossulae by all writers 

 hitherto, and the purpose of the present paper is to show the 

 true morphological significance both of them and of the axial 

 fossula. It must be admitted, however, that the axial fossula is 

 the only one corresponding with any structural peculiarity 

 (siphonoglyph) of the mature polyp. 



It is also incorrect to say that a fossula never occurs on the 

 dorsal border of the corallite in association with the dorsal-direc- 

 tive septum. As mentioned on p. 42, specimens of Zaphrcntis 

 compressa occasionally show a marked depression in this region, 

 of the same nature as that on the ventral border. 



As would be expected, Yakovleff (p. 408) is unable to accept 

 Bernard's explanation of the fossula as due to a bagging down 

 of the basal tissues consequent on the overturning of the proto- 

 theca. He also (p. 400) shows the untenable nature of Ber- 

 nard's view that the bilaterally symmetrical Rugosa have risen 

 from the radially symmetrical Madreporaria as a result of the 

 falling over of the corallite. 



o 



Yakovleff (p. 402) evidently shares the general view that the 

 fossulae are due to the presence of smaller primary septa; " Die 

 Septalgruben sind Viertiefungen, die sich bei den primaren 

 Septen befinden ... in den Septalgruben zeichnen sich die 

 primaren Septen gewohnlich durch ihre schwache Entwicke- 

 lung (geringen Dimensionen) aus." I have shown above (p. 32) 

 that though the alar fossulae are always associated with a group of 

 new smaller metasepta yet the primary alar septa (protosepta) 

 are not smaller than the other primary septa ; they form the ven- 

 tral limit of the alar fossulae, but themselves take no part in any 

 depression or enlarged interspace. Only the ventral axial fossula 

 has associated with it a smaller primary septum, and this consti- 

 tutes its fundamental distinction from the lateral depressions. 



SUMMARY. 



i. The two alar fossulae present in certain rugose corals cor- 

 respond with the region of addition of new septa within the mid- 

 dle two of the six primary interseptal spaces, and each is situated 

 on the dorsal aspect of a ventro-lateral or alar septum. The fos- 

 sula is due to the fact that some of the septa are here shorter ) 



