38 • SHELL GALLERY. 



within the valves, which serves as a nursing-pouch for the 



young-. 



Cases 162- The LucinidcB are almost invariably white shells, and may 



163. generally be recognized by the very long muscular scar in front 



on the inner surface of the valves. They occur in all parts of the 



world; and the fossil forms, which are still more numerous 



than those now living, have existed at every epoch from the 



Silurian. 



Cases 166- Of the freshwater Mussels or Unionida more than 1200 species 



177. 



Fig. 30. 



-1 '---^^^ 









British " Fan-Mussel " {Pinna pectinata) : a, the byssiis. Case 157. 



have been already discovered ; they are found in most parts of the 

 world, the greatest number having been described from North 

 America. In Unto the edges of the mantle are not structurally 

 united along the bottom and not prolonged into siphonal tubes ; 

 but are closely apposed save at the posterior end, where 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 ; 



