MORPHOLOGY OF THE MADREPORARIA. 45 



forms ; wherever continuous vertical growth takes place, without 

 the addition of new septa, radiality tends to be assumed by the 

 septa already formed. In elongated forms a considerable verti- 

 cal interval occurs between the insertion of each new bilateral 

 pair of septa and an approximate radiality is attained, to be again 

 destroyed on the addition of another pair. Somewhat similar 

 changes have been shown to be also characteristic of the growth of 

 modern hexameral corals ; the developmental stages are bilat- 

 eral, but between one stage and another an approximate radiality 

 is reached. As regards the Rugosa it may be accepted as a gen- 

 eral rule that developmental bilaterality at maturity is associated 

 with shortness of calicular form. 



It does not appear from systematic works that the presence or 

 absence of fossulae has proved to be a character of much taxo- 

 nomic value. It is conceivable that forms otherwise closely 

 allied may vary as to whether or not the septa at maturity attain 

 the radial arrangement. The facts already adduced prove, how- 

 ever, that in any series of closely allied forms those with well 

 marked fossulse represent an earlier phylogenetic stage than those 

 attaining radiality, but beyond this it does not yet seem pos- 

 sible to go. 



EXPLANATION OF THE FOSSULA. 



Concerning the significance of the fossulae the view here main- 

 tained is that they are due to two distinct structural features : (i) 

 a grouping of smaller incompletely developed septa at the region 

 of growth within the middle and ventro-lateral pairs of the six 

 primary interseptal chambers (counter and principal quadrants) ; 

 (2) in the case of the cardinal fossula only, a smaller ventral 

 directive septum correlated with the presence of a single stomo- 

 daeal groove in the polyp. The alar fossulae are altogether the 

 result of the first condition, while the cardinal fossula is either 

 dependent upon a combination of both the first and second causes, 

 or is entirely due to the second. Exceptionally, as in Anisophyl- 

 luin, the cardinal fossula is due only to the first, when it appears 

 as a double depression. 



Other explanations of the origin of the rugose fossulae have 

 been offered. The view hitherto generally accepted is that which 

 regards them as a sort of chamber for the hypertrophied mesen- 



