362 On the Prisnary SeiiUd Plan of the Rugosa. 



in a more ])rimitive wny thai that iio.v existing; these 

 considerations, however, I must defer to a future occasion. 



The last point I wish to deal with concerns tiie orientation 

 adopted for the figures in this paper. While in living corals 

 the first six or twelve septa appear simultaneously, the ento- 

 celes in which they are formed arise in bilateral pairs, of 

 which the first lateral pair appears on the dorsal surface. 

 Simihirly the dorso-lateral pair of exoceles arises before the 

 ventro-lateral one. And again, the first six tentacles of most 

 corals arise over the septa, and the dorso-lateral pair appears 

 first. It would seem to be a general rule in the Zoantharia, 

 indeed, that in the insertion of l)ihiteral pairs of this nature 

 the. first appears on the dorsal surface. The natural conclu- 

 sion is that in the llugosa the first lateral pair of septa to 

 appear, /. e. the alar septa (2.2), arose on the dorsal side of 

 the corallum, and this whether the septa were exclusively 

 entocelic or not. 



It is only fair to Duerden to remark that the reverse 

 orientation seen in his figures was adopied before the order 

 was known in which the six primary septa of the llugosa 

 appeared. 



Before concluding this paper I must express my hearty 

 thanks to Dr. A.shworth, of the University of Edinburgh, for 

 niueh valuable help, especially for the loan of papers not 

 otherwise obtainable, and to Dr. Flett and Mr. T. (J. Hall, of 

 the Geological iSurvey, for the micro-photographs on PI. iX. 



EXPLANATION OF TLAir: IX. 



Horizontal sections illustrating the formation of early septa in Rugose 

 corals. All figures have the " uuiin," " cardinal," or " i'ossular " septum 

 on the upperside, and with the exception of tigs. G a and 6 b are troui 

 different specimens. 



Fiys. 1-G& from variants of Znph. riiiUipsi, Ed. & II. 

 Fig. 1. (0. 58.) Single septum of Stage I. X 40. 

 Fii/. '2. (0.67.) Appearauce of the alar septa 2 . 2 in Stage II. 



The dark mark on the upperside is caused by earthy matter 



tilling up the vacant space left by a " Productus " spiue to 



wliich the young coral was attached — a very commou 



phenomenon in these corals, X 3o. 

 7'Vr/. ;]. (0. 90.) Early part of Stage III. The counter-lateral proto- 



septa 3. 3 ju,<t appearing. Irregular growth of septa. X 35. 

 Fi(j. A. (0.76.) End of Stage HI. The ^i\ protusepta synnuetri- 



cally arranged about the axial line. X 35. 

 Fi(j. 5. (0. 44.) Appearance of tlie lirst two metas.^ptal pairs {a a). 



The a.\ial septum lias brolvcn up into " main" and " counter'' 



septa (M and 0). X 35. 

 Fiys. G(j i^ 6 6. Both from the same specimen. <o a (C. US) shows 



