Morphohgij of the Madreporaria. 227 



equal distances apart. Also, by means of a series of micro- 

 scopic sections, I established that the subsequent principal 

 septa (meta.septa) are achled in a bilateral manner within tour 

 ot the six primary interseptal ciianil)ers, the two middle and 

 the two ventro-iatcral chambers; further, that the additions 

 are made at oidy one region within each chamber, immediately 

 dorsal to the alar or ventro-lateral septum in the case of the 

 middle chambers, and immediately next to the cardinal or 

 ventral directive septum in the ventro-lateral chambers. I 

 then proceeded to show that of all modern Anthozoa the 

 Kugosa find their nearest representatives in the zoanthid 

 aetinians. lu the Zoantheaj the secondary mesenteries 

 (metacnemes) are addeti bilaterally at one region within each 

 of the primary ventro-lateral intermesenterial chambers or 

 exocoeles, exactly as are the septa in the Rugosa, but no 

 mesenteries are added within the primary middle and dorso- 

 lateral exocoeles. From our knoiv ledge of tbe relationship of 

 the septa and mesenteries in modern corals, it was assumed 

 that the two cycles of septa of the rugose corals were formed 

 within mesenterial chambers similar to those characteristic 

 of the zoanthids, the principal or larger septa within entocoeles 

 and the secondary or smaller septa within exocoeles ; hence 

 the former are termed entosepta and the latter exosepta. 



In the sixth paper of tiiis series, published in 1905, with 

 the subtitle " The Fossula in Rugose Corals," I endeavoured 

 to show, from a series of developmental stages in Streptelastna 

 rectunij Hall, the true nature of the alar fossul^e, and also 

 that of the cardinal or ventral directive fossula. I demon- 

 strated that the latter fossula is composite in character, at 

 any rate during the early stages ; that it is made up of a 

 series of incomplete septa on each side of the ventral directive 

 septum, while the ventral directive septum is itself smaller 

 than the other principal septa. It was suggested that the 

 small ventral directive septum is to be correlated with the 

 presence in the rugose polyp of a ventral siphonoglyph or 

 gonidial groove, similar to that characteristic of modern 

 zoanthid polyps. This interpretation I considered as greatly 

 strengthening the earlier suggestion that the Rugosa are 

 nearly related to the Zoantlieie, and expressed it in the 

 following terms (p. 40) : " In the absence of the rugose polyp 

 itself, no surer proof of the relationship of the group to the 

 zoanthids coulcl, to my mind, by adduced than that which 

 admits of the correlation of the simple cardinal fossula with 

 a ventral stomodoeal groove.''' Figures were given (/. c. 

 tigs. 2-11) showing" that in Streptelasma i-ectum, a.s in Lopho- 

 phyllum^ there are six primary septa, and that the subsequent 



