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CLASS IV. BKACHIOPODA. 

 (LAMP-SHELLS.) 



THE most prominent characteristics of this Class 

 are indicated by the two appellations by which we 

 have designated them above. The name of Lamp- 

 shell refers to the close resemblance to an antique 

 terra-cotta lamp, presented by the upper or ven- 

 tral valve of one of these shells, a Terebratula t 

 for example. Here we have a bivalve, which is 

 equal-sided, but very unequal- valved, one valve 

 only having what would be called a beak in the 

 CONCHIFERA ; this projects beyond the extremity 

 of the other valve, and curves upward (if we con- 

 sider it as a lamp) or downward (if we regard the 

 natural position of the shell). This beak is per- 

 forated with a hole corresponding to that which 

 supports the wick in the lamp, but here subserving 

 the purpose of a passage for a muscular stem that 

 passes through it from the interior, and is affixed 

 to the rock or other supporting body. 



The smaller (or dorsal) valve has no beak nor 

 perforation ; it is articulated to its fellow by an 

 apparatus of teeth and sockets, allowing of a mar- 

 ginal separation to a very small extent. Gene- 

 rally both valves are convex, but sometimes the 

 dorsal valve is flat, or even very concave, as in the 

 fossil family Productadce. 



In outline the Brachiopoda most commonly 

 assume a form nearly circular, with the exception 

 of the projecting beak; but in many cases the 



