79 



Species. 



1. Cailliaudi, Fe'russ. 3. semilunata, Lam. 4. Stefanensis, Moric. 



2. elliptica, Lam. 



Figure. 



Etheria Caillaudi. PI. 30. Fig. 175. Shell of glaucous green, show- 

 ing its external tubular structure, and blistering of the internal nacre. 



Family 2. NAIADES. 



Shell ; equivalve, more or less inequilateral, pearly ', covered with a 

 thick olive-green epidermis ; hinge extremely variable, generally 

 two or more solid cardinal teeth, with or without lateral teeth, 

 sometimes toothless. 



The freshwater Mussels, which according to the latest census* number 

 539 species, form an extremely natural group, but vary remarkably among 

 themselves. The most noticeable characteristic of the animal is an un- 

 usually large development of the foot. The shell is of pearly substance, 

 covered with an olive-green epidermis ; sometimes it is thin and toothless, 

 generally it is thick and very strongly toothed. The principal home of the 

 N aides is in the great rivers of North America, but they exist in the prin- 

 cipal rivers of both the Eastern and Western hemispheres. Our own island 

 has four species. 



Mr. Lea divides the family into two genera, to which he gives the names 

 of Margaron and Platiris ; and these he divides into nine subgenera. Under 

 Margaron he has Triquetra (which answer to Hyria and Prisodon), Unio, 

 Margaritana, Honocondylaa, and Dipsas, which five subgenera constitute 

 our Unio ; and he has Anodonta. Platyris comprises Iridina and Spatha, 

 which we include in one, and Mycetopus. Our genera will therefore stand 

 thus : — 



Unio. Hyria. Anodonta. 



Iridina. Mycetopus. 



Genus 1. UNIO, Retzius. 



Animal ; thick and fleshy, with the mantle freely open ; foot large 

 and broadly compressed. 



* Lea, Synopsis. 



