434 MOLLUSCA: BKACHIOPODA. 



The shell of the Brachiopods is composed of two 

 valves, and occupies a dorsal and ventral position in rela- 

 tion to the body of the animal. 



The valves are of unequal size, the dorsal being the 

 smaller, and the ventral valve the larger. 



The dorsal valve is always free, and not perforated ; the 

 ventral has a more or less prominent beak, through which 

 passes a fleshy peduncle, by means of which the animal is 

 attached to submarine bodies. 



Unlike the shells of the typical mollusks, the valves are 

 symmetrical in shape, the two sides being alike. 



The two valves are articulated by two curved teeth de- 

 veloped from the margin of the ventral valve, and received 

 by sockets in the dorsal, making a hinge so complete that 

 the two valves cannot easily be separated. 



The Brachiopods are divided by naturalists into two 

 Orders : 



1. ARTHKOPOilATA, as Terebmtala, Spirifer, etc. 



2. LYOPOMATA, as Litigula, Distinct, etc. 



