MINERALS AND GEOLOGY OF CANADA. 



109 



Bryozoa. — The bryozoons (so named from the general moss-like 

 aspect of their united cells) are minute animals of marine existence. 

 They form cell-colonies after the manner of most coral animals, but 

 present a higher organization than these latter. They possess a dis- 

 tinct oral and anal cavity, and assimilate in many other respects to 

 the molluscous type. The compound cell-structure in some forms 

 takes the shape of leaf-like expansions, and in others is either den- 

 dritic, plumose, rounded, or irregular. It is also either free, or 

 attached by growth to shells and other sub-marine bodies. 



Modern bryozoons abound in all seas. Fossil forms of this class 

 are also exceedingly numerous, ranging throughout the entire series of 

 fossiliferous rocks. Their separation from corals is in many instances, 

 however, a task of much perplexity ; and, as those found in our Cana- 

 dian strata are of little importance as test- 

 forms, we confine our illustrations to a 

 single example, Fenestella elegans, (Fig. 

 87), from the Niagara Group of the Upper 

 Silurian Series. Representatives of the 

 class, it may be observed, occur as low 

 down as the Calciferous-Sand-Rock (see 

 Part V.) ; and Professor Dawson, on the 

 other hand, has found a number of species 

 identical with existing forms, in the Post- 

 tertiary deposits of Eastern Canada. These are described in the 

 4th volume of the Canadian Naturalist. 



The GraptoUtes, already described as a section of the Polypifera 

 or Corals, (see Vol. VI., p. 503) are referred by some palaeontologists 

 to the present class. 



Brachiopoda. — The brachiopods are marine, headless mollusks, 

 provided with a bivalve shell. The valves of this shell are always of 

 unequal size ; and one is situated on the dorsal, and the other on the 

 ventral side of the animal. The ventral valve is almost invariably the 

 larger of the two, and without reference to the anatomy of the mol- 

 lusk would be naturally taken for the dorsal valve. The valves, 

 though unequal in size, are " equilateral " — i. e., a vertical line 

 drawn straight through the middle of each valve, divides the shell 

 into two exactly equal parts. This serves to distinguish at a glance 

 a brachiopod shell from the shells of other bivalves : or at least from 

 the great majority of these, as some few, the Peciens for example, 



Fi^. S7. 



