COl^CEYUA—DITHYRA. xlv. 



B. Testae aquarum dulcium, 53—56. 



Fresh water Shells, 



Independently of their general systematic classification, 

 the fresh water bivalve shells form a series naturally distinct 

 from those of the ocean, by their locality and habits. They 

 have therefore been brought together, as exhibiting a division, 

 the separate study and arrangement of which may be rendered 

 of more easy and simple accomjjlishment. 



All the natives of the British Islands have the Ligament 

 external, and are comprehended in four genera, whose essential 

 characters may be thus exemplified : 



53. Anodon, Cardo edentulus, sinu antico marginali. 



Hinge without teeth, with a marginal depression 

 at the anterior side. 



54. Unio, Cardo dentibus primariis, lateralibus nullis. 



Hinge with primary teeth, but no lateral ones. 



55. Mysca, Cardo dentibus primariis, lateralibus anticis. 



Hinge with primary teeth, and lateral ones on 

 the anterior side only. 



56. Cyclas. Cardo dentibus duobus primariis, 'lateralibus utrinqui. 



Hinge with two primary teeth, and lateral ones 

 on each side. 



