48 



SHELL GALLERY. 



placed on each side of the aniraal, as in the 

 bivalve niollusks, but are respectively dorsal 

 or ventral or upper and lower. In Tere- 

 bratula, Rhynchonetta, &c. they are locked 

 together by a kind of hinge, which is so 

 constructed that it is almost impossible to 

 separate them without injury. Crania, Lin- 

 gula, and others have no hinge, the valves 

 being merely held together by muscles. Bra- 

 chiopods are found in all seas, attached to 

 rocks, corals, and other bodies. They are 

 found at any depth, in pools at low-water 

 mark or in 2900 fathoms. Only about one 

 hundred and forty species of living Brachiopods 

 are now known ; but at former periods they 

 were much more numerous, over 1800 extinct 

 forms having been described. 



Fig. 33. 



' 



Duck's-bill Lingula 

 (Lingula anatina). 

 (From the Indian 

 Ocean.) 



