CAMAROTCECHIA ACINUS HALL (PAGES 339-341) 



FIGURE 9. Ventral view of a young shell in a secondary stage of 

 growth ; showing a single growth-line and the embryonal median ridge. 



FIGURE 9a. Dorsal view of the same. The foramen is slightly nar- 

 rowed, but without evidence of deltidial plates; the embryonal sinus is 

 well defined. 



FIGURE 9b. Outline profile of the same. 



FIGURE 10. Ventral view of a specimen 3.25 mm. in length. 



FIGURE 10a. Dorsal view of the same. The mature fold and sinus 

 have not yet begun to develop ; the foramen shows increased constriction 

 at its base, and slightly thickened margins. 



FIGURE 10ft. Outline profile of the same. 



FIGURE 11. Dorsal view of an average adult, x 4. 



FIGURE lla. Profile of the same. 



FIGURE 116. Front view of the same; showing the elevation of the 

 median fold. 



RHYNCHOTRETA CUNEATA DALMAN, var. AMERICANA 

 HALL (PAGES 351-356) 



See Plate XXII 



FIGURE 12. Dorsal view of a shell representing the earliest stage of 

 growth observed ; length 1.5 mm. The foramen is widely triangular and 

 unobstructed, the surface is covered with all the plications of maturity, 

 and the sub-circular valve bears a broad median depression. 



FIGURE 12a. Outline profile of the same. 



FIGURE 13. Similar view of a shell 3 mm. in length. The outline 

 has become elongate, the foramen narrowed, and its margins thickened. 



FIGURE 13a. Outline profile of the same. 



FIGURE 14. Dorsal view of a normal adult individual; showing the 

 elevated beak and broad plications. Natural size. 



FIGURE 14a. Profile of the same; showing the elevation of the dor- 

 sal fold. 



FIGURE 146. Front view of the same. (28^ Rept. N. Y. State Mm. 

 Nat. Hist., pi. 25, figs. 31, 34, 36.) 



FIGURE 15. A series of outlines of the anterior margin ; showing 

 the embryonal sinus or depression in the dorsal valve in young forms, 

 reaching its maximum in individuals having a length of 4.5, and becom- 

 ing obsolete, or the line of junction straight, in specimens 7 mm. in 

 length. Outline 6 shows the inception of the dorsal fold. In 8 the four 

 median plications are distinctly elevated ; 9 is from a normal full-grown 

 individual, and 10 represents the maximum elevation of the fold. The 

 outlines, except in 9 and 10, are enlarged to the diameter of a fully 

 developed specimen. 



FIGURE 16. Cardinal area of figure 12. 



FIGURE 16. Outline profile of the same. 



