5<3 J. E. DUERDEN. 



like manner. Although in the great majority of rugose corals 

 the ventral-directive septum (Hauptseptum), along with its asso- 

 ciated fossula, occurs on the convex border, yet Yakovleff finds 

 the relationship to be occasionally reversed. Such differences 

 the author considers to be a mechanical necessity in the formation 

 of the secondary septa, according as to whether the mouth or oral 

 surface of the living polyp was inclined to one side or the other 

 of the corallite. In the majority of cases it is supposed that the 

 oral disc was inclined towards the concave side, whereas when 

 the ventral directive septum is on the concave side, the polypal 

 disc was directed towards the convex border. 



From Yakovleff' s account it is evident that various authors 

 have been misled as regards the orientation of the corallite by 

 assuming that the concave and convex borders are morphologi- 

 cally the same throughout. As he points out, the only reliable 

 criteria for purposes of orientation are the relationships of the 

 septa among themselves. Though the axial fossula may occur 

 on either the convex or the concave side of the corallite, it is 

 always associated with the cardinal or ventral-directive septum ; 

 the ventral border may be either convex or concave, or indeed 

 at any angle to the concavo-convex axis, according to the species 

 or even individual. 



Yakovleff investigates rather fully the claim which has been 

 made for the occurrence of three or four fossulas in certain 

 species. With regard to Omphyma he shows that the presence 

 of more than one' depression is doubtful, as also in the genus 

 Menophyllum. He concludes for the Rugosa in general that 

 only one fossula, the ventral-directive, is really determinable, the 

 others resting upon insufficient evidence. In support of this he 

 refers to Bernard's statement, already quoted, that he fails to see 

 the evidences for the existence of more that one true fossula in 

 any coral. 



In connection with this I would say that whether the alar 

 region with its group of smaller septa, and more or less special 

 interspace, be included within the term fossula is merely a matter 

 of definition. In the forms already referred to (p. 44) such 

 special regions among the septa do occur, and correspond with 

 developmental stages of others in which they are absent at 



