Figure 17. — Cardinal area of figiiro 13. 



FiGUKK 17a. — Outline proiik- ut" liie same. 



FiGUKK 18. ^ Cardiiuil area in an individual 4.5 mm. in length; sliow- 

 ing the considerably advanced development of thedeitidial plates and the 

 encroachment of the foramen on the apex of the valve. 



FiGUKE 18a. — Outline profile of the same. 



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



FiGUKE 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 

 length 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. 



