into three parts. The cross-lines, representing the natural size of the 

 specimen, are too short. 



FIGURE 8a. Outline profile of the same. 



FIGURE 9. Cardinal view at a size of 12 X 11 nun. The ventral 

 valve and area have become greatly curved, and the dorsal aperture is- 

 quite filled by the tripartite cardinal process. 



FIGURE 9a. Outline profile of the same. 



FIGURE 10. Cardinal view of a large adult; size 18 X 18.5 mm. 

 The areas are closely appressed, and the dorsal aperture is wholly filled 

 by the cardinal process, the central portion of which extends into the 

 aperture of the other valve. 



FIGURE 10a. Outline profile of the same. 



FIGURE 11. Cardinal view of a normal adult; natural size. 



FIGURE lla. Profile of the same. (op. cit., pi. 21, figs. 17, 14.) 



FIGURE 12. Dorsal view of a small adult. 



FIGURE 12a. Ventral view of the same. (op. cit., figs. 12, 11.) 



RHIPIDOMELLA IIYBRIDA SOWERBY (PAGES 321, 322) 



FIGURE 13. Cardinal view of a very young individual having a length 

 of 1 mm. and a width of 1.5 mm. The valves are nearly equi-convex, the 

 area and apertures as in the earlier stages of D. elegantula. 



FIGURE 13. Outline profile of the same. 



FIGURE 14. Cardinal view of a somewhat gibbous example measur- 

 ing 10 X 8 mm. The relatively short areas are about equally developed, 

 and the cardinal callosity of the dorsal valve has already filled the dorsal 

 aperture. 



FIGURE 14a. Outline profile of the same. 



FIGURE 15. Cardinal view of a normal adult 12 X 10 mm. ; showing 

 the short area and the projection of the cardinal process into the ventral 

 aperture. 



FIGURE 15a. Outline profile ; showing the incurvature of the areas. 



FIGURE 16. Dorsal view of a large adult, (op. cit., fig. 20.) 



FIGURE 17. Profile of a normal adult, (op. cit., fig. 21.) 



FIGURE 18. Dorsal view of a small, gibbous example ; showing- 

 strong varices. Enlarged to two diameters. 



FIGURE ISa. Profile of the same. 



