J. E. DUERDEN. 



sisi, E. & H., the alar septa are much larger than any subsequently 

 developed, and may thus emphasize the fossula adjacent to them. 

 On the other hand, it is very doubtful if the alar septa themselves 

 are ever smaller than the other principal septa, or are situated 

 within the fossular depression, as is sometimes assumed in palae- 

 ontological works. 



Alar fossulae of a like nature with those in Hadropliylluui 

 pauciradiatum have been studied in other tetracorallids, especially 



in Microcyclus discus, Meek 

 & Worthen, Anisophylluin 

 Agassizi, E. & H., and also 

 in Hadrophyllum glatis 

 (White). In these there is 

 the same grouping of suc- 

 cessively smaller septa, giv- 

 ing to the alar region its 

 distinctive character. A 

 still larger number of za- 

 phrentoid corals, however, 

 are found to resemble 

 Strcptclasina rectum : * dur- 

 ing the earlier stages of 

 development their new 

 septa are related one to 

 another just as they are in the adult of Hadrophyllum, while on 

 approaching maturity they become equal and more strictly radial ; 

 they all, in fact, pass through a Hadrophyllum stage. 



The true morphological significance of the difference between 

 adult forms like Hadrophyllmn with alar fossulse and others like 

 S. rectum in which they are wanting is thus only ontogenetic. 

 In the one development does not proceed sufficiently far as to 

 establish approximate radial symmetry, and the adult corallite 



1 It may be well to state that in the course of the investigations the septal develop- 

 ment has been followed by means of serial sections in various species of the following 

 genera comprised within the families Cyathaxonidae, Palreocyclidje, and Zaphrentidse, 

 namely, Cyathaxonia, Dmicnnella, Pahaocychts, Hadrophylluin, Microcyclus, 

 Slreptelasmct, Zapkrentis, Lophophylliitn, and Anisophyllum. Representatives of the 

 families Cyathophyllidre and Cystiphyllidre, particularly the colonial forms, are 

 found to be unsuitable for such studies. 



FIG. 5. A second pair of septa (/>) is 

 now present, one in each of the middle cham- 

 bers, inclined towards the metasepta (a). 



