77 



The two genera associated under this head have little real affinity, and 

 ought rather to be taken as the types of distinct families. Chama is a 

 vigorous growing, profusely ornamented sea mollusk ; Etheria is a dull 

 freshwater bivalve, inhabiting the rivers of North Africa and the Amazon. 

 The hinge is of very rude structure, consisting of no more than a blunt 

 oblique tooth, which is sometimes obsolete. 



Genus 1. CHAMA, Linnceus. 



Animal ; with the mantle closed, the margins being united by a 

 filamentary fringe, except at the siphoned orifices, which are 

 small and distant ; foot cylindrical, bent. 



Shell ; orbicular, ovate, or oblong-ovate, irregular, profusely orna- 

 mented with spines, scales, or lamella, loioer valve more or less 

 deeply convex, upper valve fatter ; umboes unequal, distant, in- 

 voluted sometimes to the right, sometimes to the left, in the same 

 species. N 



Excepting the Oysters, there is no genus of bivalves whose shells are 

 liable to so much variation of form and general aspect, according to cir- 

 cumstances of habitation, as Chama. Being strictly of parasitical habit, and 

 inhabiting the most confined situations in crevices of rock and fissures of 

 madrepore, without the power, possessed by terebrating mollusks, of re- 

 moving the obstacles to their growth, the shells are mostly more or less 

 distorted, according to their place of attachment. Yet notwithstanding 

 this, they are characterized by vivid colouring and a most luxuriant growth, 

 developed in all sorts of varieties of spines, scales, and fronds. The spe- 

 cies are difficult of definition, and the difficulty increases when it is found 

 that the same species varies materially in growth in different habitats. 

 When allowed to develope its foliations in placid water, surrounded by 

 food most favourable for the secretion of colour, the shell is hardly to be 

 recognized as the same stunted form that has had to contend with the buf- 

 feting of the storm. 



Above fifty species are known from Central America, Australia, and the 

 Pacific and Eastern Seas; and some are found to be reversed. The same 

 species may be observed with its umboes involuted sometimes to the right, 

 sometimes to the left. 



Species. 



1. appressa, Reeve. 3. aspersa, Reeve. 5. Broderipii, Reeve. 



2. arcinella, Linn. 4. brassica, id. 6. Carditseformis, id. 



