BRACHIOPODA. 271 



Holland, from their destructive attacks upon the wood 

 of the flood-gates and dykes. A few weeks' immer- 

 sion of a piece of fir-wood, suffices to enable the 

 Teredo to bore it through and through, and even the 

 liardest oak is not able to resist this formidable de- 

 stroyer. 



CHAPTER XVII. 



Fourth Class of Mollusc a. 

 Brachiopoda.* 



This is a very limited group, the members of which 

 might readily be supposed at first sight to belong to 

 the ordinary bivalves described in the last chapter. 

 They are contained within a pair of shells, more or less 

 resembling those of the common cockle. One shell, 



FjG. 208. — FIGURE OF BKACHIOPOD. 



however, is larger and more convex than the otlier, 

 and is generally pierced with a hole near the hinge. 

 The shells are for the most part fixed to some rock 

 or other object by a fleshy stalk, but in one genus 

 (Orhicida) the lower valve itself is cemented to the 

 rock. 



* ^pax'icoi^, brad lion, an arm ; irovs, ttoSos, pous, podos, a foot — 

 arm-footed. 



