FiGUKK 10a. — Outline profile of tlie same. 



Figure 11. — Dorsal view of au adult specimen, somewhat al)ove the 

 average size, and enlarged to 1|- diameters; showing the noiuial ieaturcs 

 oi' maturity. 



Figure 11a. — Trofile of the same. 



Figure lib. — Ventral view of the same imlividual. Natural size. 



Figure 12. — Transverse section of an indi\ idiial ; showing tlic sjiiral 

 ribbon and the number of volutions. 



Figure 13. — The character of the i)edicle-i)assago in the youngest, 

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

 the foramen are thickened. 



Figure 13a. — Outline protilf of the same. 



Figure 14. — A later stage of growth, in which the ]ilates are consid- 

 erably developed. 



Figure lia. — Outline profile of the same. 



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

 circular and apical. The plates are slightly ilexctl along two oiili(jue 

 lines which converge toward the base of the area. 



Figure I'xi. — Outline profile of the same. 



Figure 1G. — The mature condition of the pedicle-area, the deltidial 

 plates being somewhat concealed b}' the incurvature of the ventral 

 umbo. 



Figure IQa. — Outline profile of the same. 



Figui-es lo-lGa are di'awn to a scale. 



ATRYPINA DISPAlllLIS IIai.i. (Packs 3(10 372) 



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

 and pedicle characters essentially as at maturity. The sinal plications 

 appai'ent at this early stage appear, in the usual adult form, as the broad 

 median fold. 



Figure 17a. — 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 gn>ater nundter of lateral plications 

 than in the normal adult (enlarged). 



Figure 10, — Dorsal view of a normal adult. 



Figure 19r/. — Ventral view of the same. 



Fkjure 10/>. — Profile of the same. All are enlarged to two diameters. 

 (28/// Repf. N. Y. Slate Mns. Nat. Ilht., pi. 25, figs. '39-11.) 



Figures 20-23. — Enlarged views of the umbonal region of the ven- 

 tral valve; showing the variations of form and ]i()sition in the fnramrn 

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



