FIGURE 10a. Outline profile of the same. 



FIGURE 11. Dorsal view of an adult specimen, somewhat above the 

 average size, and enlarged to 11 diameters; showing the normal features 

 of maturity. 



FIGURE lla. Profile of the same. 



FIGURE 116. Ventral view of the same individual. Natural size. 



FIGURE 12. Transverse section of an individual ; showing the spiral 

 ribbon and the number of volutions. 



FIGURE 13. The character of the pedicle-passage in the youngest 

 example (figure 10). The deltidial plates are absent, but the margins of 

 the foramen are thickened. 



FIGURE 13. Outline profile of the same. 



FIGURE 14. A later stage of growth, in which the plates are consid- 

 erably developed. 



FIGURE 14a. Outline profile of the same. 



FIGURE 15. A stage of growth in which the foramen has become 

 circular and apical. The plates are slightly flexed along two oblique 

 lines which converge toward the base of the area. 



FIGURE 15a. Outline profile of the same. 



FIGURE 16. The mature condition of the pedicle-area, the deltidial 

 plates being somewhat concealed by the incurvature of the ventral 

 umbo. 



FIGURE 16a. Outline profile of the same. 



Figures 13-16a are drawn to a scale. 



ATRYPINA DISPARTLIS HALL (PAGES 369-372) 



FIGURE 17. A young individual 2.5 mm. in length ; showing surface 

 and pedicle characters essentially as at maturity. The sinal plications 

 apparent at this early stage appear, in the usual adult form, as the broad 

 median fold. 



FIGURE 17. Outline profile of the same; showing the erect beak 

 and shallow valves. 



FIGURE 18. A young individual ; showing asymmetry in the devel- 

 opment of the median fold, and a greater number of lateral plications 

 than in the normal adult (enlarged). 



FIGURE 19. Dorsal view of a normal adult. 



FIGURE 19. Ventral view of the same. 



FIGURE 19&. Profile of the same. All are enlarged to two diameters. 

 (2Sth Rept. N. Y. Slate Mus. Nat. Hist., pi. 25, figs. 39-41.) 



FIGURES 20-23. Enlarged views of the umbonal region of the ven- 

 tral valve; showing the variations of form and position in the foramen 

 as observed among specimens which differ but little in actual size. 



