PLATE V 

 Retzia evax, Hall 



Page 55 

 S«e Plate viii 



V'lR. t. Dorsal view of the younf^est individual observed, having a length of i mm. and a width of .8 mm. The ventral umbo 



is erect, the foramen tiiangularand without deUidial plates; the surface bears two faint lines of growth, and out^de the 



second of these, are three fine plications on each side of a median sinus. 

 Fig. irt. Outline profile of the same; showing the very shallow valves. 

 Fig. 3. Dorsal view of an average adult, showing the characters of advanced growth. 

 Fig. tut. Profile of the same. (28th Rept. X. Y. State Mus. Nat. Hist., pi. 25, figs. 14, 18.) 

 Fig. 3. Dorsal view of an immature in<lividual, 5 mm. in length, in which the plications on the earlier portion of the shell end 



abruptly at a growth-line, from there outward the surface characters being altogether primitive. 

 Fig. 3ia. Outline profile of the same. 



Fig. 4. 'I'he cardinal area in its earliest obser^'ed condition ; enlarged from fig. x. 



Fig. 5. A later stage of growth in the cardinal area, the deltidial plates having a considerable development. 

 Fig. 6. A still later condition of this area, in which the plates have united, enclosing the foramen. 

 Fig. 7. The cardinal portions of an individual with an unusually elevated ventral umbo; showing also an advance in growth 



from the condition represented in fig. 6. 

 Fig. 8. The character of the cardinal area in a normal adult of about the size represented in fig. 2. The fofamen has become 



circular and the ventral umbo so incurved as to conceal the deltidial plates. 

 Fig. 9. A small obese example in which the foramen is almost wholly concealed. 



Retzia sobrina, sp. n. 



Page 61 

 Fig. 10. The youngest shell observed, having a length of 2 mm. and a width of i.6mm. The shell already bears two plications 

 on each side 6f the nyedian silif s, andlvyo touch f«1nter cl6[vated sXtix fnthe.fiinusltve^. ' / ' 

 Outline profile of the same. ' ' - ^ ^ ! / . 



, Dorsal view of an adult specimen, somewhat above the average size, and enlai^ed to i^ diameters; showing the normal 

 features of maturity. 

 Profile of the same. 



Ventral view of the same individual; natural size. 



Transverse section of an individual; showing the spiral ribbon and the number of volutions. 



The character of the pedicle-passage in the youngest example (fig. 10). The deltidial plates are absent, but the margins 

 of the foramen are thiclcened. 

 Fig. jyt. Outline profile of the same. 



Fig. 14. A later stage of growth, in which the plates are considerably developed. • 



Fig. 14/1. Outline profile of the same. 

 Fig. X5. 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. 

 Fig, 15a. Outline profile of the same. 



Fig. 16. The mature condition of the pedicle-area, the deltidial plates being somewhat concealed by the incurvature of the ven- 

 tral umbo. 

 Fig. i6a. Outline profile of the same. 



The drawings i3-x6a are drawn to a scale. 



CCELOSPIRA DISPARILIS, Hall 



Page 64 



Fig. 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. 



Fig. 170. Outline profile of the same; showing the erect beak and shallow valves. 



Fig. z8. A young individual; showing asymmetry in the development of the median fold, and a greater number of lateral plica- 

 tions than in the normal adult (enlarged). 



Fig. 19. Dorsal view of a normal adult. 



Fig. 19a. Ventral view of the same. 



Fig. 19*. Profile of the same. All are enlarged to two diameters. (28th Rept. N. Y. State Mus. Nat. Hist., pi. 25, figs. 39-4X.) 



Figs. 30-33. Holarged views of the umbonal region of the ventral valve; showing the variations of form and position in the foramen 

 u observed »mong spedmens which differ but little in actual size. 



