BEACHIOPODA. . 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 

 hardest oak is not able to resist this formidable de- 

 stroyer. 



CHAPTER XVII. 



FOURTH CLASS OF MOLLUSCA. 

 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 BRACHIOPOD. 



however, is larger and more convex than the other, 

 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 

 (Orbicula) the lower valve itself is cemented to the 

 rock. 



* /3pax*W, bracliion, an arm ; TTOUS, TroSos, pous, podos, a foot 

 arm-footed. 



