OF CONCHOLOGY. 55 



adhere to rocks in deep water, and 

 require considerable force to tear 

 them from their retreats. 



Some of the Spond}'li have ears, 

 and others not ; they are in general 

 strong, rugose shells. This genus re- 

 quires considerable attention, as some 

 of the species vary exceedingly, so 

 much so that many have been taken 

 for distinct species. 



The French writers divide the Spon- 

 dyli into two genera : Spondylus, and 

 Plicatula. The valves of the last are 

 in general rude, thick, uneven, and 

 uncouth. 



In Turton^s Linne only four spe- 

 cies are described ; no species of this 

 genus have been found to inhabit the 

 British seas. 



Genus 12th.--CHAMA. Animal sl 

 Tethys ; shell bivalve, rather coarse ; 

 hinge with a callous gibbosity, ob- 

 liquely inserted into an oblique cavity; 



