PLATE XXI 



MERISTINA MARIA HALL (PAGES 377-380) 



FIGURE 1. An embryo measuring .75 X .75 mm. ; without deltidial 

 plates. 



FIGURE la. Outline profile of the same. 



FIGURE 2. A later stage of growth, at which the shell measures 

 5x5 mm., and the deltidial plates have developed sufficiently to give 

 the foramen a circular outline. 



FIGURE 2a. Outline profile of the same. 



FIGURE 3. Dorsal view of a normal, mature shell. 



FIGURE 3a. Profile of the same. (28th Ann. Rept. N. Y. State Mus. 

 Nat. Hist., pi. 25, figs. 9, 12.) 



MERISTINA RECTIROSTRIS HALL (PAGES 372-374) 



FIGURE 4. The cardinal features at an extremely early stage of 

 growth, the shell measuring 3x2 mm. ; showing the broad, triangular 

 foraminal opening and its faintly thickened margins. 



FIGURE 4a. Outline profile of the same. 



FIGURE 5. The same features when the shell has attained a size of 

 4.5 x 3.5 mm. ; showing the gradual approximation of the sides of fora- 

 men and the narrowing of the umbo, without the formation of deltidial 

 plates. 



FIGURE 5a. Outline profile of the same. 



FIGURE 13. The same features at maturity. 



FIGURE I3a. Outline profile of the same. 



FIGURE 11. Dorsal view of a young individual. 



FIGURES 12, I2a, 12ft. Dorsal, profile, and ventral views of an adult 

 specimen, (llth Ann. Rept. GeoL Surv. Ind., pi. 27, figs. 1, 10-12.) 



WHITFIELDELLA NITIDA HALL (PAGES 374-377) 



FIGURE 10. Dorsal view of the youngest individual observed; hav- 

 ing a size of 2.5 X 1.75 mm. 



FIGURE !(). Outline profile of the same. 



FIGURE 6. Cardinal features of a shell having dimensions of 3 X 2 

 mm. 



FIGURE 6a. Outline profile of the same. 



FIGURE 7. The same features in an individual having a size of 

 0x4 rnm. In this example the umbo is abnormally elongate, giving an 

 unusual prominence to the deltidial plates. 



FIGURE 7a. Outline profile of the same. 



FIGURE 8. The same features at a size of 5X4 mm. ; showing the 

 total concealment, from incurvature, of the deltidial plates. 



FIGURE 8a. Outline profile of the same. 



FIGURES 9, 9a, 9ft. Dorsal, ventral, and profile views of the normal 

 adult. (28th Ann. Rept. N. Y. State Mus. Nat. Hist., pi. 25, figs. 3, 4, 5.) 



