PLATE XXI 



MERISTINA MARIA Hall (Pages 377-380) 



Figure 1. — An embryo measuring .75 X .75 mm. ; without deltidial 

 plates. 



FiGur.E In. — Outline profile of the same. 



FiGUKE 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. 



FiGUHE '2((. — Outline profile of the same. 



Figure 3. — Dorsal view of a normal, mature shell. 



Figure 3a. — Profile of the same. (28;/; Ann. Rept. N. Y. Stale Miis. 

 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 S x ^ 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 featm-es at maturity. 



Figure 13a. — Outline profile of the same. 



Figure 11. — Dorsal view of a young individual. 



Figures 12, 12a;, 12?). — Dorsal, profile, and ventral views of an adult 

 specimen. (11(7; Ann. llept. Geol. Sun:. Ind., pi. 27, figs. 1, 10-12.) 



WPIITFIELDELLA 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 10«. — Outline profile of the same. 



Figure G. — Cardinal features of a shell having dimensions of 3 X 2 

 mm. 



Figure 6a. — Outline profile of the same. 



FiGURK 7. — The same features i]i an individual having a size of 

 0X4 mm. In this example the umbo is abnormally elongate, giving an 

 uimsual prominence to the deltidial plates. 



Figure 7a. — Outline profile of tlie same. 



Figure 8. — The same features at a size of 5 x 4 mm. ; showing the 

 total concealment, from incurvature, of the deltidial jilates. 



Figure 8a. — Outline profile of the same. 



Figures !), 9a, 96. — Dorsal, ventral, and profile views of the normal 

 adult. (2.S//i Ann. Ilfjit. N. Y. State Hhis. Nat. Ilisl., pi. 25, figs. 3, 4, 5.) 



