MORPHOLOGY OF THE MADREPORARIA. 41 



cal differentiation associated with its extremities. In this respect 

 living coral polyps differ from the closely allied skeletonless 

 hexactinians, where a gonidial groove usually occurs at each 

 extremity of the stomodseum. 



An examination of other species of rugose corals demonstrates 

 that the cardinal fossula is somewhat similarly constituted 

 throughout the group. The two extremes of its development 

 are the complex condition met with in forms like Hadrophyllum 

 and the simple modification present in the mature calice of 

 Streptelasma. In the one we have it associated with a grouping 

 of incompletely developed septa on each side of a feebly de- 

 veloped axial septum, while in the other it is represented by the 

 smaller axial septum alone. The fossula in the first has an 

 ontogenetic as well as a morphological significance, while in the 

 second it is correlated with a structural peculiarity of the mature 

 polyp. In zaphrentoid corallites in which the calcareous material 

 has been replaced by silica, and the septa are exposed throughout 

 their vertical length, 1 the entire series of developmental changes 

 undergone by both the alar and the cardinal fossulat can some- 

 times be observed at a glance, confirming the results obtained 

 from a series of sections like that shown in Figs. 2-1 I. 



The condition in rugose corals with the newly added septa 

 arranged bilaterally in four groups, distinct from the other septa, 

 I propose to term the Hadrophyllum-stage, since it is so charac- 

 teristically displayed in this genus. In general it will be associ- 

 ated with three fossulae. In its significance the stage is com- 

 parable with that of the Edwardsia or Halca)npa-sta.gQ met with 

 in some mature actinians and the polyps of modern corals, and 

 passed through in the ontogeny of others. All the simple 

 Rugosa, namely those embraced within the group Zaphrentoidea, 

 exhibit the Hadi'ophyl/iun-s\.a.gQ for a longer or shorter period 

 during their development ; in some it is retained at maturity, 

 while in others it is replaced by a more nearly radial condition 

 of the septa. 



1 For beautifully perfect silicified specimens of Streptelasma profiinJinn (Owen), 

 with all the interseptal matter removed by decalcification, I am indebted to Prof. Y. 

 W. Sardison, and for others from the geological collections of the University of Michi- 

 gan to Prof. I. C. Russell. For many other species of Rugosa from the National 

 Museum I am under obligations to Prof. C. Schuchert, and from the British Museum 

 to Dr. F. A. Bather. 



