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EXPLANATION OF THE PLATES. 



Plate XIX. 



Fig. 1. Cyclas calyculata ; Showing the general appearance of the shell and animal 



inhabitant in that genus. 

 Fig. 2. Pisidium amnicum. Ditto. 

 Fig. 3. Hinge of Cyclas calyculata. A, the right, and B, the left valve; a, the 



cardinal teeth. 

 Fig. 4. Hinge of Pisidium amnicum. A, B, and a, as before. 



Plate XX. 



Fig. 1. Pisidium obtusa/e. 



Fig. 2. Ditto ; as it appears when walking on the under side of the surface of the water. 



Fig. 3. Ditto; viewed from one end. 



Fig. 4 and 5. Pisidium pusillum. Two extreme varieties. 



Fig. 6. Ditto; viewed from one end. 



Fig. 7. Pisidium nitidum. 



Fig. 8. Ditto ; viewed from one end. 



Plate XXI. 



Fig. 1. Pisidium pulchellum: a, b, and c, different appearances of the siphonal 



tube. 



Fig. 2. Ditto, Var. /3: d, e, f, different appearances of the siphorial tube. 



Fig. 3. Ditto ; viewed from one end. 



Fig. 4. Ditto, Var. $. 



Fig. 5. Ditto; viewed from one end. 



Fig. 6. Pisidium Hensloivianum. 



Fig. 7. Ditto ; viewed from one end. 



Fig. 8. Ditto, young. 



Fig. 9. Young; viewed from one end. 



Obs. All the above figures are highly magnified. 



