10 



fig. 13) of the same age (ready to emerge from the egg-case) had 

 one more lateral and two narrow flat marginal plates. The 

 radula of a mature specimen (Plate 2, fig. 14) measures 1.2 mm. 

 in length and 0.3 mm. in width and contains 110 rows of teeth. 

 The radula of the mature specimen is therefore nearly four times 

 as long and five times as broad as that of a specimen ready to 

 leave the egg-case, and the individual teeth are approximately 

 three times as large, although identical in shape. The remarkable 

 difference between the radulae of the mature and the young is in 

 the number of lateral teeth, the mature having 20 to 30 laterals, 

 the young 3 to 4. 



At no stage during the development is there any sign of a spiral 

 twist to the animal or to the shell. That portion of the mantle 

 supporting the shell assumes a flat disk-like shape (Plate 2, fig. 6) 

 early in the development of the embryo and retains that shape and 

 position until the shell is formed and the animal leaves the egg- 

 case. 



EXPLANATION OF PLATE 2. 



Fig. 1. Ventral view of two individuals in coitu. X3. 



Fig. 2. Egg-case containing seven capsules, each with a single egg. X12. 



Fig. 3, a, b, c. Segmentation of the egg. X40. 



Fig. 4. Embryo on seventh day. X40. 



Fig. 5. Embryo on eighth day. X40. 



Fig. 6. Embryo on ninth day, lateral view showing mouth and buccal mass. 

 X40. 



Fig. 7. Abnormal egg-case containing 5 eggs. X25. 



Fig. 8. Embryo on tenth day, ventral view. X40. 



Fig. 9. Embryo on tenth day, dorsal view. X40. 



Fig. 10. Shell one day after leaving egg-case. X40. 



Fig. 11. Jaw of animal one day after leaving the egg-case. X350. 



Figs. 12, 13. Teeth of radula of animal one day after leaving the egg-case. 

 X3000. 



Fig. 14. Teeth of radula of mature specimen. X1000. 



