PLATE XVII 

 STEOPIIONELLA STRIATA Hall (Pages 330-334) 



Figure 1. — Ventral view of the incipient sliell of the series ; length 

 2.25 mm. ; showing the opening of the pedicle-sheath, and the primary 

 surface plications. 



FiGUKE 1(1 — Outline profile of the same ; showing the complete con- 

 vexity of the ventral valve, and essentially conformable concavity of the 

 dorsal valve. 



Figure 2. — Ventral valve of a normal adult; showing the umbonal 

 convexity of the shell and general concavity over the pallial region. 

 (28(/j Rept. N. y. State Mus. Nat. Hist., pi. 23, fig. 1.) 



Figure 2a. — Outline profile of the same ; showing the reversal in 

 convexity from the embryonic condition. 



Figure 3. — Pedicle-area of the specimen represented in figure 1 ; 

 length 2.25 mm. ; showing the well-developed, slightly exsert sheath 

 and the obscure dorsal callosity. 



Figure 4. — Pedicle-area of an individual 2.5 mm. in length, in 

 which the sheath is extravagantly exsert. 



Figure 5. — Pedicle-area in an example about 6 mm. in length. 



Figure 6. — Pedicle-area, when a length of 8 nnn. has been attained. 



Figure 7. — Pedicle-area in a shell measuring 13 mm. in length. 



Figure 8. — Pedicle-area in a normal adult measuring 17 mm. in 

 length. 



Figures 3-8 have been drawn to the same scale, and show the succes- 

 sive phases in the development of the pedicle characters. The sheath 

 ceases its function as a pedicle-passage before maturity is attained, 

 though retaining its relative size ; while tlie dorsal callosity, which in the 

 earlier stages is grooved and largely enveloped by the sheath, is eventu- 

 ally separated from the sheath by a narrow aperture, and its surface 

 becomes uninterrupted. 



MIMULUS WALDRONENSIS ]\[ii,ler and Dvkr (Pages 334, 335) 



Figures. — Dorsal view of a young individual having a length of 

 3 mm. The .«;hell is nearly symmetrical, and .shows an open triangular 

 deltidium, ending in a sub-circular apical foramen. 



Figure i)ii. — Ventral view of I he sanir ; showing the apical foramen. 



Figure dh. — Outline profile of the same. 



