40 SHELL GALLERY. 



with radiating or concentric ribbing, and usually are coated with a 

 dark epidermis. They have the general appearance of certain 

 Veneridce ; but the animal has no prolonged siphons, but merely a 

 fringed opening in the mantle. One very remarkable species, 

 ThecaUa concamerata, has an internal cup-like process within the 

 valves, which serves as a nursing-pouch for the young. 



Asiphonida. 



TCases Of the freshwater Mussels or Unionidce more than 1200 species 



21c-22h.J have been already discovered ; they are found in most parts of the 

 world, the greatest number having been described from North 

 America. In Unio the edges of the mantle are not united along 

 the bottom and not prolonged into siphonal tubes ; at the posterior 

 end there are two openings, of which the upper or excretal orifice 

 is simple, and the lower or branchial fringed at the edge. The foot 

 is very large and adapted for crawling and burrowing. The sexes 

 are distinct; and the shells of the females are somewhat more tumid 

 than those of the males. Margaritana miirgaritifera, which is 

 found in this country and in Europe, sometimes produces handsome 

 pearls, but not equal to those obtained from the pearl-oyster of 

 tropical seas. 

 rp The family JEtheriida, or freshwater Oysters, consists of but 



23 a.] three genera : Attheria contains African, and Mulleria and Bartlettia 



South-American forms. When young the shells of JEtheria (which 

 are common in the Nile) are free and not unlike an Anodonta, but 

 when adult they become attached and irregular and look like an 

 olive-green Oyster; they are, however, provided with two mus- 

 cular impressions instead of one, as in ordinary marine Oysters. 

 Still more remarkable is Mulleria of New Granada, which, when 

 young, freely moves about and has two adductor muscles, but in 

 time becomes attached and stationary, and then possesses but a 

 single adductor. 



r _, ..„ The Mi/tiUdce, or Mussels, are too well known to need descrip- 



[Case 23, / ■ ■ • • 



a-e.] tion. The small foot, which is brown in the common species, is 



not much used in creeping about, but has the power of spinning 



abyssus or bundle of tough threads, by means of which the animals 



attach themselves to rocks and one another, forming colonies of vast 



numbers. Mussels have always been much eaten in this and other 



