32 



Length -f diam. -^ axis T 5 ^ of the breadth. 

 This species is more common than the preceding, 

 and smaller, usually one inch in breadth ; teeth 

 broad ; edge sharp. 



Var. 1. fusca, almost entirely brown. 



Var. 2. vermiculata, with flexuous brown 

 transverse lines. 



Fifth Genus OBLIQUARIA. 



Form variable, often somewhat transverse, and 

 more or less oblique posteriorly ; ligament 

 oblique ; cardinal tooth bilobed, commonly fur- 

 rowed ; lamellar tooth oblique, often straight ; 

 axis variable ; marginal contour thickened ; three 

 muscular impressions ; animal similar to that of 

 the Unio. 



This group is numerous in species ; it differs 

 principally from the Unio or Elliptio in its form, 

 by the ligament and by the oblique lamellar 

 tooth, &c. It presents many anomalies, and secon- 

 dary characters, which oblige me to divide it 

 into six sub-genera. 



First Sub-genus PLAGIOLA. 



Axis extramedial ; lamellar tooth curved ; liga- 

 ment curved; form of the shell variable, but not 

 oblique. 



