OF CONCnOLOGY. 41 



Fig. 4. Jaw of )S. tristensis. 



a, anterior view. b, posterior view. 

 Fig. 5. Jaw of S. thersites from in front. 



Fig. 6. S'q)honaria tristensis from beloAv, in tlie Bhell. 



Fig. 7. The same from above with the shell removed and the 

 mantle turned back. 



m, attachment of the mantle to the shell. 



r, renal organ. 



h, cardial sac cut open to show the heart. 



p, pulmonary papilla and lung traversed by blood vessels. 



b, branchia. 



The dotted line indicates the course of the rectuiHv 



