MOLLUSCA — PELECYPODS 



229 



There are more than one thousand Uving species. In the 

 fossil state the genus is especially well represented in the Cre- 

 taceous of the Rocky Mountains. 



(LT, 



a.a.<c.t. 



B 



Fig. 98. — A fresh water mussel, Unio liiteoliis Lamarck, from Chisago Lake, Min- 

 nesota. A, exterior of Uving animal, left side. B, interior of the conjoined 

 valves; a. a., anterior adductor muscle scar; a.r., scar of anterior foot retractor 

 muscle; c.t., cardinal teeth; e.s., excurrent siphon; i.s., incurrent siphon; /., 

 ligament; ma., mantle edge; p.a., posterior adductor muscle scar; umb., umbo, 

 showing here its characteristic corrosion by the acids in the water. Natural 

 size. 



1. Sketch interior view of one valve, noting teeth and sockets, 

 pallial line, ligament and its place of insertion. 



2. In the opening and closing of the valves what function 

 was performed by (a) the teeth and sockets ; {b) the ligaments ; 

 (c) the muscles ? 



3. What is the habitat of Unio? Its common name? 



4. Why is the chitinous exterior of this form thicker than 

 that of marine forms ? 



