BEYOZOA. 607 



cation, they are brought close together. Though developed less 

 numerously and shorter than they are in T. ramulosus and 

 divaricans, it is now evident that the total absence of dissepi- 

 ments can no longer be insisted upon as one of the distinguish- 

 ing features of Thamniscvs, but we must fall back upon the 

 frequent dichotomization of the branches as the only remaining 

 diagnostic character. Of course the relative paucity of the dis- 

 sepiments aid very materially in separating species of the 

 genus from those of Polypora, yet a sharp divisional line be- 

 tween these two groups is out of the question. In fact, I think 

 that a very nearly complete line of development can be shown 

 between T. dubius and Polypora in which P. dendroides (McCoy) 

 P. gracilis (Prout) T. divaricans and T. ramulosus would figure 

 prominently. The question, what about the Silurian and De- 

 vonian species which have been placed under Thamniscus? ', I 

 cannot enter into at present, beyond the simple expression of 

 my belief that none of them belong to the special line of devel* 

 opinent that culminated in T. dubius, unless, perchance, we 

 agree that the tendency to deviate in that direction was re- 

 tained by certain forms which may have reverted to the parent 

 branch, and only required the proper condition to again assert 

 itself. It does not, however, seem at all probable that all of 

 them were subjected to this possible reversion, since in T. sculp- 

 tilis and T. octonarius, we have two species that are most 

 probably descendants of the more ancient line. Some of the 

 Devonian species complicate the geneological problem by pre- 

 senting marked external resemblances to Idmonea, but how far 

 the agreement may be the result of true relationship remains 

 to be determined. 



For the present we my say that Thamniscus, as now under- 

 stood, commences in the Niagara with one species,* is repre- 

 sented by two species in the Lower Helderberg, continues through 

 the various Devonian and Carboniferous groups, and culminates 

 in the Permian formation. Neither the species, of which the 

 total number is about sixteen, nor the individuals seem at any 

 time to have been abundant. 



* Hall's T. niagarensis belongs to my new genus Drymotrypa. but his Hornera? 

 dichotoma, is closely allied to the Wenlock Thamniscus crassus Lonsdale sp. 



