MORPHOLOGY OF THE MADREPORARIA. 



35 



in an alar fossular group (c-c), that is, the shorter septa are 

 turned towards the successively larger, and are united with them 

 by their inner ends in a unipinnate fashion. In general also the 

 space on each side between the group of newly added septa and 

 the axial septum is greater than the other septal interspaces and 

 more pit-like. 



Again comparing the adult calice of Hadropliyllnm with the 

 developmental series of sections of Strcptdasnia in Figs. 2-1 1, 



a. 



FlG. 8. A stage somewhat in advance of that in Fig. 7. An additional septum 

 (</ ) is present in each middle chamber, while the two pairs, c, are much larger. The 

 section shows very clearly the inclination of the septa towards each other in the alar 

 and ventro-lateral regions, only the oldest septa reaching the columella (cf. Figs. I 

 and 12). 



it is seen that the fossular region on each side of the cardinal 

 septum is a region where the addition of new septa takes place, 

 just as is that on the dorsal aspect of each alar septum. Like- 

 wise, the new ventral septa in Sfreptclasma are inclined towards 

 the older and fused with them until the mature condition is 

 reached, when they become free, of the same size as the others, 

 and more nearly radially arranged (Figs. 10, 11). Therefore the 

 cardinal or ventral groups of shortened fused septa in Hadro- 



