282 Brown — Developmental Stages in Streptelasma rectum. 



This criticism seems altogether unwarranted, since Duerden 

 does not produce a single species from the Ordovicie showing 

 the primary hexameral arrangement. His Duncanella from 

 the Siluric has six pairs of septa in the earliest stages obtain- 

 able, and his statements concerning Streptelasma rectum from 

 the Devonic are incomplete and misleading. The only species 

 for which primary hexamerism has not been disproven are 

 from the Devonic and Carbonic, very late in the geological 

 distribution of rugose corals. 



The statement stands unquestioned that a type occurring 

 late in geological time, at least a considerable time subsequent 

 to the earliest occurrence of a type at all similar, is likely to 

 be far from primitive in at least some respects. 



That the counter quadrants of a rugose coral are accelerated 

 in development over the cardinal quadrants is shown by the 

 above discussion of Streptelasma rectum. One tertiary sep- 

 tum has appeared in each counter quadrant before even a sin- 

 gle secondary septum has appeared in the cardinal quadrants. 

 Three secondary septa appear in each counter quadrant before 

 the appearance of the first secondary septum in the cardinal 

 quadrants. In all seven secondary septa appear in each of the 

 counter quadrants, while only four come in the cardinal quad- 

 rants. Duerden's statement that " the smaller septa (exosepta) 

 [tertiary septa] arise almost simultaneously at a rather late 

 development stage, and are thus of no significance as regards 

 septal sequence,"* is somewhat too sweeping a statement. In 

 Streptelasma rectum these do not arise simultaneously but 

 come in in the same order as the secondary septa. The first 

 one in each counter quadrant appears long in advance of any of 

 the others, and when the others do appear they follow the same 

 sequence as the secondary septa. They develop more rapidly 

 in the counter quadrants than in the cardinal, four having 

 appeared in the former when there are only two in the latter. 



It therefore appears that the presence of six primary septa 

 in rugose corals and subsequently developed tetramerism is not 

 established. For this species at least primary tetramerism has 

 been proved and the probability is that all the rugose corals 

 are primarily tetrameral and the appearance of six septa in the 

 early stages of geologically late species is due to the early 

 development of the first pair of secondary septa. 



After the above paper was completed, my attention was 

 called to a paper by Mr. R. C. Carruthers in the November num- 

 ber of the Annals and Magazine of Natural History on " The 

 Primary Septal Plan of the Rugosa." In this paper the author 

 reviews briefly the principal papers on the question of a prim-. 



* Ann. and Mag. of Nat, Hist., loc. cit., p. 240. 



