190 



PALAEOZOIC TIME LOWER SILURIAN. 



2. Calciferous Epoch. 



The great magnesian limestones of this epoch in the Mississippi yalley rarely 

 contain a fossil, and only a few species occur in the Calciferous beds of New 

 York. Species are, however, numerous in some of the limestone layers of the 

 Quebec group, and already 137 have been described from these strata. 



Protozoans. — Sponges? — Archeocyathus Minganensis B. occurs at the Min- 

 gan Islands, in the lower part of the Calciferous. 



Radiates. — a. Polyps. — No Polyp-corals have been found, unless the Archeo- 

 cyathus belongs to this group. The genus Stenopora is represented among 

 the Canada beds and at the Mingan Islands, and a doubtful coral from Phillips- 

 burgh is referred by Billings to Tetradium. But both are probably genera of 

 Acaleph-corals, — that is, the stony secretions of Hydroid Acalephs (see p. 162). 

 The square form and four rays of the cells of Tetradium suggest this reference 

 for that genus ; and it is required for Stenopora by the minute size of the cells 

 as well as the relation of the corals to the tabulate Favosites and Pocillopora. 



b. Acalephs. — Graptolites* are common in the shales of this epoch, and 42 

 Canada species have been described by Hall. Figs. 246-248 represent the 

 Graptolitlius Logani H. from Canada, showing the centre of a group and the fur- 

 cating mode of branching. They are supposed by Hall to have been spread 



* The annexed figures illustrate the relations of the Graptolites to living spe- 

 cies of animals. The fossils are never calcareous ; the texture is membranous 

 or a little horny, and usually only faint impressions are left in the rocks. No. 1 

 represents one of the branching Bryozoans, the Notamia loricata, and 1 a the 



Fig. 252 A. 



2 



same enlarged. No. 2 is the Sertularia abietina, and 2 a the same enlarged. 

 Along the branches there are capsules containing bulbs for reproduction ; the 

 bulbs pass out from the capsule when mature. No. 3 represents another species,, 

 the Sertularia rosacea, and 3 a the same enlarged ; 2 a has one of the bulb- 

 bearing capsules. 



The prominent distinction in the corallum of the Bryozoans and Sertularians 

 is that in the former the cells have no internal tubular connection, while in the 

 latter the axis of the stem is tubular, and all of the cells communicate with one 



