ETHEEIA. 311 



in Upper Nubia. They are described by the latter in liis 

 ' Voyage a Meroc/ as being a common article of food^ and 

 their shells are collected by the natives in order to decorate 

 the tombs of deceased relatives. 



Eiheria, though once considered as somewhat allied 

 to the Naiades, differ materially in their habits and mode 

 of growth^ being found in small clusters. They adhere 

 firmly to one another, and present the most singular distor- 

 tions of form, the nacre lining the interior being generally 

 of a livid green colour, often raised in small blisters, and 

 not iridescent. 



No genus of acephalous mollusks, excepting perhaps the 

 oyster, are liable to such variations in form and general 

 aspect as the Chama. All parasitic genera are affected to a 

 certain extent by irregularities in their places of attach- 

 ment, but the shells of which we spetik are mentioned by 

 Mr. Eeeve as more than usually distorted, by their inhabit- 

 ing the crevices of rocks or corals, without an inherent 

 power of abrading, or else absorbing, the obstacles to their 

 natural advancement. Thus, a great difference subsists 

 between a specimen which has grown uninterruptedly on a 

 smooth surface, and one that has been formed within a very 

 limited area, subject to external pressure ; as also betweeen 



