Brown — Developmental Stages in Streptelasma rectum. 281 



In the above discussion it is definitely shown that in Strep- 

 telasma rectum only four septa are present in the earliest 

 stages and that all others come in later, therefore it is con- 

 tended that Duerden is wrong in claiming primary hexamerism 

 for all rugose corals. The possibility of proving primary hex- 

 amerism in such disc-like forms as Hadrophyllum and Micro- 

 cyclns is questioned, and the presence of six septa in the ear- 

 liest recognizable stages of Lophophyllum proliferum (if a 

 still earlier stage cannot be obtained), can easily be explained 

 as a differential development, that is, by the theory of accel- 



eration or retardation of one part of an organism with respect 

 to the rest of the organism.* 



Duerden states that : " This idea of acceleration is altogether 

 hypothetical, and its author does not produce a single accepta- 

 ble fact in its support. He considers that a departure from 

 the original tetrameral type is likely to occur in a form such as 

 Lophophyllum, which appears in Carboniferous times, that is, 

 towards the close of the geological distribution x>i the rugosids. 

 This argument, unsatisfactory in itself, now fails altogether in 

 view of the fact that comprised in the list of corals given 

 above, in which six primary septa have been definitely estab- 

 lished, there are representatives of almost all ages in the 

 chronological extension of the rugose corals."f 



* See C. E. Gordon, this Journal, vol. xxi, pp. 109-127, Feb., 1906. 

 f Ann. and Mag. of Nat. Hist., Inc. cit., p. 239. 



