146 ANNALS NEW YORK ACADEMY OF SCIENCES 



largement of the thecse and an oblique tendency of the unsupported rhab- 

 domes, which leads to a bryograptoid exterior. 



Gerontastic features are noticeable in some specimens of Dictyonema 

 flabelliforme var. conferta, in which the development of the main varietal 

 feature is carried to an extreme as exhibiting a reduction of the diameter 

 of the synhabdome on its posterior end. Very large individuals of var. 

 acadica indicate an undulating shape of the rhabdome, while this is tele- 

 scoped in other specimens into ephebastic and even neanastic stages. 



As to the possibility of keeping distinct the young stages of Dictyo- 

 nema and Staurograptus, the 4-branch-stages of the first one, which does 

 not go beyond the declined 19 position, and the 6-branch-stage of Stauro- 

 graptus, which gets into a deflexed or even horizontal position of the 

 branches, are of greatest importance. A similar difference holds true in 

 regard to Bryograptus, of which Westergard has recently described beau- 

 tiful specimens of B. hunnebergensis ((4), pi. 5, figs. 10-23). 



I may call attention to the fact that the Dictyonemas here found ex- 

 hibit all features of a true epacmic genus, since this increasing period of 

 evolution is indicated by (1) the relatively long duration and the clear 

 distinction of the early stages, because of non-existence or at least only 

 the beginning of acceleration of the characters; (2) the appearance of 

 new "postspecific" features in the metephebastic age, which may become 

 inherent ones and may be pushed back by acceleration in the descend- 

 ants; (3) the fact that senile features make their appearance late and 

 are rare, if we confine them to those which doubtless manifest a decline 

 of the individual or the colony; (4) the fact that the genus in question 

 maintains its position at the base of a more or less widely branching 

 evolutional series. All these features are shown in Dictyonema flabelli- 

 forme from Navy Island, 1ST. B. 



On the Structuee of Dictyonema flabelliforme 



Though the specimens available for study were compressed upon the 

 shale and so do not permit investigations similar to those made by 

 Wiman, I have nevertheless been compelled to direct my attention to the 

 points raised by these investigations, since several recent writers have 

 spoken rather slightingly of them or have considered them open to 

 question. 



At first glance, a striking difference seems to exist between the grapto- 

 litic mass, one rhabdome exhibiting a reflecting silvery surface while 

 others show a matt blackish one. By cautiously scraping on the best- 



19 See Ruedemann (1), p. 485. 



