TABULAR SCHEME. 67 



Hinge with few or no teeth 



Shell almost circular Axinus, 66-68. 



Shell inequilateral 



Ligament external and large 

 Teeth conspicuous Unto, 55-57. 

 Teeth inconspicuous Anodonta, 58, 59. 



Ligament internal 



With radial ribs, transverse plates, and one 

 upright tooth ; hinge margin crenulated 

 behind the ligament Crenella, 24, 25. 

 With two groups of strias from beaks 

 Modiolaria, 26-29. 



Beaks terminal and pointed 



With shelf within the beak Dreissensia, 60. 

 Without shelf Mytilus, 19-23. 



B. Pallial line not indented ; siphons short 



Grooves radial Cardium, 129-138. 



Grooves concentric 



Beaks twisted Isocardia, 54. 



Beaks not twisted 



Hinge without teeth Galeomma, So. 



Hinge with teeth 



Shell oblong Lcpton, 76-79. 



Shell triangular 



Two teeth in each valve Astarte, 50-52. 

 Three teeth in each valve, one cloven so as to make 

 the three look like four Circe, 113. 



Shell circular or sub-orbicular 



Ligament or cartilage internal 

 Anterior scar long and extending within pallial 



line Loripes, 63, 64. 

 Anterior scar orbicular Montacuta, 69-72. 



Anterior scar oval 



Left cardinal small Las&a, 75. 



Left cardinal thick and erect Kellia, 73, 74. 



Ligament more or less external 



Shell circular ; two teeth in each valve, anterior 

 in left and posterior in right, being bifid ; 

 ligament double Diplodonta y 65. 



