FiGDEK 19. — A young example with a length of 6 mm. ; showing the 

 inception of the median fold, (lltfi Ann. Re]:!. Geol. Surv. Intl., pi. 27, 

 fig- 6.) 



Figure 20. — Dorsal view of a larger example, having two plications 

 on the fold, and abnormal in the absence of all lateral plications. 

 Natural size. (op. cit,, fig. 5.) 



Figure 21. — Dorsal view of a small, essentially mature example, 

 with two plications on the fold. (op. cit., fig. 4.) 



Figure 22. — Dorsal view of an adult with three plications on the fold. 

 (28«A Rept. N. Y. Slate Mus. Nat. Hist., pi. 26, fig. 13.) 



Figure 23. — Front view of an adult with four plications on the fold. 

 {op. cit., fig. 22.) 



Figure 24. — Dorsal view of a similar individual. 



Figure 24a. — Profile of the same. {op. cit., figs. 15, 19.) 



Figure 2.5. — An enlargement of the cardinal area in an individual 

 with a length of 1.5 mm, ; showing essentially the primitive characters 

 seen in figure 17. 



Figure 25a, — Outline profile of the same. 



Figure 26. — The condition of the ventral cardinal area and foramen 

 in an individual 3.5 mm. in length. The margins of the foramen are 

 thickened by the inception of the deltidial plates, and the aperture is seen 

 to encroach upon the apical portion of the shell. 



Figure 26a. — Outline profile of the same. 



Figure 27. — The cardinal features in an individual of 7 mm. length. 

 The advance upon the last stage is chiefly in rapid development of the 

 deltidial plates, which have narrowed the opening, slightly constricting 

 it near the now incurved umbo. 



Figure 27a. — Outline profile of the same. 



Figure 28. — The cardinal features in an adult, having a length of 

 10 mm. The deltidial plates have united at their base, forming an elon- 

 gate oval aperture, encroaching upon the umbo. The species does not 

 pass this stage of development. 



Figure 28a. — Outline profile of the same. 



