Brown — Developmental Stages in Streptelasma rectum. 283 



ary hexameral or tetrameral condition in the Rugosa, referring 

 especially to the recent publications before cited by Professor 

 J. E. Duerden and Professor C. E. Gordon. As a result of 

 his studies he has come to the conclusion that Gordon's obser- 

 vations are correct, but in the light of his (Carruther's) inter- 

 pretation of those observations they do not contradict but 

 rather help to support Dnerclen's contention that the primary 

 stage of the rugose corals is a hexameral one. 



In his studies on a small Zaphrentid found abundantly in 

 the Carboniferous shales of Scotland, Carruthers has found that 

 there are three stages of development that a rugose coral 

 passes through before the six primary septa are formed and 

 take their places as equally developed septa. 



These stages he describes as follows : 



" Stage I. — A single septum is seen to stretch across the 

 calicle from wall to wall. This may conveniently be referred 

 to as the 'axial septum.' In later stages this axial septum 

 breaks up to form the main and counter septum of the mature 

 coral. . . . 



" Stage II. — Two new septa are next seen to arise, one on 

 each side of the ' main ' end of the axial septum. Though 

 remaining attached to the wall of the calicle and to the axial 

 septum, they gradually spread outwards, and eventually form 

 the ' alar ' primary septa of Kunth. 



" Stage III. — Shortly after the alar primaries have devel- 

 oped another pair appears, in the same manner as before, but 

 at the opposite or ' counter ' end of the axial septum. These 

 also spread outwards, though very rarely to the same extent as 

 the alar septa. There is now a distinct pause in the formation 

 of new septa, and no more appear for some time. . . ." 



These careful observations and the plate illustrating the 

 paper prove, as the present writer has attempted to prove in 

 the above paper, that an earlier stage than the six septa stage 

 of Duerden can be found in the rugose corals. The only ques- 

 tion now is as to the interpretation of the observed facts. Mr. 

 Carruthers' observations differ from those of the writer in one 

 point, namely, the manner of coming in of the second pair of 

 lateral septa, which he calls the counter lateral or second lateral 

 pair of primary septa, and which I call the first pair of second- 

 ary septa. He has observed this pair of septa to first appear 

 as very short septa, one en either side of the counter or '•■ axial" 

 septum, and as they developed and enlarged the point of attach- 

 ment moved along the median septum until they became radi- 

 ally or nearly radially placed. Strange as it may seem, this is 

 the very method by which in my first studies I attempted to 

 derive this pair of septa, but with the material at hand could 

 not find individuals that showed this manner of growth. In 



*Ann. Mag. Nat. Hist., 7th ser., vol. xviii, No. 107, pp. 356-863, Nov.. 

 1906. 



