Figure 17. — Cardinal area of figure 13. 



FiGUUE 17((. — Outline profile of the same. 



FiGUKE 18. — Cardinal area in an individual 4.5 mm, in length ; show- 

 ing the considerably advanced development of thedeltidial plates and the 

 encroachment of the foramen on the apex of the vah e. 



Figure 18a. — Outline profile of the same. 



Figure 19. — Cardinal area at a length of 5 mm. 



Figure 19a. — Outline profile of the same. 



Figure 20. — Cardinal area at a length of 7 mm. 



Figure 20a. — Outline profile of the same. 



Figure 21. — Cardinal area in an individual which has attained a 

 lengtli of 12 mm. The plates are united for nearly one-half their length, 

 and slightly bent outward about the base of the foramen. 



Figure 21a. — Outline profile of the same. 



Figure 22. — The cardinal area at maturity; showing a stronger 

 flexion of the plates below the foramen, and a sub-circular foramen en- 

 croaching upon the apex of the shell. 



Figure 22a. — Outline profile of the same. 



