FAMILY 13. CHAMACEA. 127 



Senegal, and by Cailliaud in the Nile, as high up as the cataracts of 

 Robatas in Upper Nubia. They are described by the latter in his ' Voy- 

 age a Meroe' as being a common article of food, and their shells are said 

 to be collected by the natives for the purpose of decorating their tombs. 

 The discovery of these curious mollusks at a considerable distance from 

 the sea, entirely refuting the postulate observations of Lamarck, induced 

 some interesting notices from De Ferussac, Cailliaud and Rang, the last 

 of whom describes them as being somewhat allied to the Naiades. They 

 differ, however, materially in their habits and manner of growth : being 

 found in small clusters, they adhere firmly to each other, presenting the 

 most singular distortions of form ; and the nacre lining the interior, 

 which is not iridescent, is generally of a livid green colour, often raised 

 in small blisters. The periostraca or outer coating is sometimes covered 

 with irregular short spines ; but this is inconstant, and varies in indivi- 

 duals of the same species. 



The shell of Etheria may be described as being irregular, inequivalve, 

 fixed, and covered with a thick periostraca, which is generally much 

 eroded. The interior is somewhat pearly, of a glaucous green colour, 

 but not iridescent, and the nacre is sometimes raised in small blisters, 

 said to be formed when the mantle of the animal becomes irritated by the 

 accidental introduction of small particles of sand during the secretion of 

 the nacreous fluid. The umbones are very short, and almost lost in the 

 increase of the shell. The hinge, which is entirely destitute of teeth, is 

 smooth, unequal, and somewhat sinuous. The muscular impressions are 

 of an oblong form, and one of them is often partially obliterated. The 

 ligament is external, but twisted, and partially penetrating within. 



The genus Mulleria of De Ferussac was described from a specimen of 

 Etheria, in which one of the muscular impressions was probably obli- 

 terated. 



Example. 



PL XCV. Fig. 1 and 2. 



Etheria semilunata, Lamarck, Ann. du Mus., pi. 32. f. 1 and 2. 

 Anim. sans vert., new edit., vol. vi. p, 595. Deshayes, Enc. Meth. 



