FIGURE 17. Cardinal area of figure 13. 



FIGURE 17a. Outline profile of the same. 



FIGURE 18. Cardinal area in an individual 4.5 mm. in length ; show- 

 ing the considerably advanced development of the deltidial plates and the 

 encroachment of the foramen on the apex of the valve. 



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 

 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. 



