THE MOLLUSCS OF THE GREAT AFEICAN LAKES. 201 



Fig. 5. — Male Nassopsis, showing the mucous gland, tentacles, and 

 genital aperture. 



Tig. 6. — Mantle cavity of female Nassopsis, showing character of gill 

 and osphradium. 



Fig. 7. — The nervous system of Nassopsis dissected from above. 



Fig. 8. — Dissection of Nassopsis, showing relations of stomach, crysta- 

 line style, kidney, oesophagus, and intestine. 



Fig. 9. — The radular elements of Nassopsis. 



Fig. 10. — Nervous system of Nassopsis dissected on the right side. 



Fig. 11. — Details of the alimentary tract of Nassopsis. 



Fig. 12. — The relations of the buccal mass, radular sac, and salivary glands 

 of Nassopsis. 



PLATE 21. 



Fig. 1. — The relation of the heart, pericardial cavity, and reuo- pericardial 

 connection in Nassopsis. 



Fig. 2. — The reproductive apparatus of a male Nassopsis. 



Fig. 3. — Two views of the shell of By th oceras Ho wesii. 



Fig. 4. — Young shell of Bythoceras Howesii, showing the absence of 

 the spines above and below the moulh. 



Fig. 5. — Dissection, showing the gill and mantle organs of Bythoceras. 



Fig. 6. — Tiie nervous system of Bythoceras dissected from above. 



Fig. 7. — The buccal mass, radular sac, and salivary glands of Bythoceras. 



Fig. 8. — The nervous system of Cerithium vulgatum dissected from 

 above. 



Fig. 9. — The radular elements of Bythoceras. 



Fig. 10. — The nervous system of Bythoceras dissected from the right 

 side. 



Fig. 11. — Right side view of the buccal mass, salivary glands, and radular 

 sac of Bythoceras. 



Fig. 12. — The operculum of Bythoceras. 



Fig. 13. — Details of the alimentary apparatus of By thoceras. 



