132 University of Michigan 



PLATE A'lII 



The scales represent lengths of one millimeter. 



Fig. 20. Stoastomops walkeri. The type specimen from B4. 



Fig. 21. Stoastomops walkeri. Inner view of operculum; the cal- 

 careous portion is viewed through the horny plate. 



Fig. 22. Tudora maculata. The type, from B3a, with operculum 

 in place. 



Fig. 23. Tudora maculata. Median view of operculum. 



Fig. 24. Tudora maculata. Outer surface of the operculum. 



