OF CONCHOLOGY. 
41 
Fig. 4. Jaw of S. tristensis. 
a, anterior view. b, posterior view. 
Fig. 5. Jaw of S. thersites from in front. 
Fig. 6. Siphonaria tristensis from below, in tbe sbell. 
Fig. 7. The same from above with thè shell removed and thè 
mantle turned back. 
m, attachment of thè mantle to thè shell. 
r, renai organ. 
h, cardial sac cut open to show thè heart. 
p, pulmonary papilla and lung traversed by blood vessels. 
b, branchia. 
The dotted line indicates thè course of thè rectum. 
