MORPHOLOGY OF THE MADREPORARIA. 



2 9 



in their" Coralliaires " (III., p. 334), specially distinguish as hav- 

 ing three fossulne, one axial and two lateral, and may therefore be 

 taken as showing the alar fossulae in their typical condition. In 

 the figure given each alar fossula is seen to be associated with 

 a few smaller septa (c-c) which fall short of the center, the short- 

 ening varying successively in a gradual manner. Further, each 

 of the shorter septa is inclined dorsally by its inner border 

 towards the next larger septum and is fused with it, so that 

 together they form a group of septa very distinct from the per- 

 fect septa, and separated from the 

 alar septum by a somewhat deeper 

 and wider interspace. 



Frequently, however, between 

 the primary alar septum and the 

 group of incomplete septa on its 

 dorsal aspect there is scarcely any 

 special depression or pit present, 

 such as is implied in the term fos- 

 sula ; even individuals of the same 



i , t FIG. 2. Primary stage with six 



species vary much in this respect. 



equal septa and six mterseptal 



Hence it will give a more precise spa c e s or chambers. The axial 

 morphological significance to the 

 term if it be extended so far as to 

 refer to the smaller, grouped con- 

 dition of the septa, whether or not the four lateral septa are the dorso- 

 they are separated by a special lateral and v entro-lateral pairs, the 

 depression from the alar septum. 

 The subsequent discussion of the 

 alar fossulae will therefore have 



septa are the dorsal and ventral 

 directive septa, the former being 

 the Gegenseptittn and the latter the 

 Haiiptscptitni of German writers ; 



latter being the Seitensepta or alar 

 septa. The two middle interseptal 

 chambers are the counter quadrants 

 of palaeontologists, and the two ven- 

 f , tro-lateral are the principal or chief 



reference more to the group of 



quadrants. 



smaller septa in this region than 



to any actual depression in the calice with which they may or 

 may not be associated. Where the grouping occurs it is always 

 a conspicuous feature of the calice. 



The significance of the shorter septa associated with the alar 

 fossula in Hadrophyllum can be understood by comparing the series 

 of figures (Figs. 2-11) representing the complete septal develop- 

 ment of Streptelasina rectum, as revealed by successive sections of a 



