TABLE OF GENERA. 69 



II. Bivalves. Shell formed of two valves, connected 

 together by a ligament on the dorsal edge. 



* Shell with diverging hinge-teeth ; inside not pearly. 



33. Cyclas. Shell oblong, nearly equilateral. 



34. Pjsidium. Shell ovate, inequilateral. 



* * Shell with irregular hinge-teeth ; inside pearly. 



37. Unio. Shell with distinct posterior lateral laminar teeth. 

 36. Alasmodon. Shell without any lateral teeth. 



* * * Hinge toothless. 

 35- Anodon. Shell ovate, pearly; umbones (dorsal) blunt. 



38. Dreissena. Shell triangular, not pearly ; umbones (anterior) 



acute. 



In describing shells, they should be regarded in their 

 natural position ; that is to say, in the way in which 

 they are placed on the animal ; thus, the part of the 

 shell over the head of the animal is called the front, and 

 that over the tail the back, of the shell ; and the left 

 and right sides of the shell correspond with the left 

 and right sides of the animal. 



This is exceedingly easy to be determined in the 

 univalve shells, because the apex of the shelly cone, 

 whether it be simply conical or spiral, in all univalves 

 (except Patella and Lottia) is over the hinder part of 

 the animal ; therefore, if a shell is placed on its mouth, 

 with the apex towards the spectator, the parts of the 

 shell will correspond with the position of the person 

 who is looking at it. 



As all shells are formed of a shelly cone (which, 

 when very long, is generally rolled round an imaginary 

 axis, for the purpose of diminishing the space that 



