OF CENTRAL CANADA PART IV. 



229 



[n addition to these forms, several more or less related fossil-types 

 are commonly referred to the graptolites. As 

 regards Canada, the Dictyonema is the princi. 

 pal representative of these doubtful types. 

 It forms dark, usually carbonaceous, undula- 

 ting, recticulated markings, as shewn in fig- 

 ure 134, which represents a common species, 

 D. retiformis, of our Niagara and Clinton 

 Formations. By some palseoutologists it is 

 regarded as a Bryozoon, by others as a sea 

 weed. 



Fio. 134. 



Dietyonema retiformis : Hall 

 Upper Silurian. 



HYDKOCORALLA. 



This sub-division is to some extent a group of convenience, ren- 

 dered necessary by our still uncertain knowledge of its included 

 forms. Some of these are undoubted Hydrozoa with tabulated cal- 

 careous corallum. The tabulated character of this corallum with 

 other features, as the absence (or rudimentary nature) of radiating 

 septa, &c. compels the collocation in the same group, as maintained 

 by Agassiz, of many of the " Tabulata " of older groupings thus 

 separating these latter from the typical " Hexamerous Corals " with 

 which they are still so commonly placed, although from the general 

 absence of septa it is impossible to tell whether the tentacles of the 

 living animal were hexamerous or not. These tabulata again, offer 

 a complete transition into the allied although in classifications 

 usually widely separated Rugosa, in many of which the assumed 

 tetramerotis character of the septa is not recognizable. Some of the 

 forms placed under the present division may perhaps be Bryozoa, 

 and others, again, Alcyonarians : but it is not possible to determine 

 this. On the other hand, the tabulated structure which they possess 

 in common, serves to unite them conveniently and, in default of 

 negative evidence, naturally also, into a common group, under the 

 name of Hydrocorolla. This name indicates their affinities to the 

 Hydrozoa, on one side, and to the corals proper, on the other.* 



* The classification of corals into Hexacoralla and Octocoralla, althongh definite enough 

 in certain cases, is on the whole entirely conventional. In many forms there are no visible 

 septa ; and in many in which well-developed septa are present, the actual number is exceed- 

 ingly variable. The genera Stylina, Lam., Stylocoenia, Edw. and Haime, Heterophyllia, McCoy, 

 and numerous others, are examples. 



