EXPLANATION OF PLATE IL 



Fig. 5a. — External structures of left valve of Plagiola sectiris (Lea)- 

 Male shell reduced one-fourth from natural adult size. 



3 

 1-2 



1-3 

 2-4 



— Extreme ventral point. 

 — Extreme anterior point. 

 — Extreme posterior point. 

 — Antero-ventral section. 

 — Post-ventral section. 

 — Antero-dorsal section. 



3-4 

 5 

 6 



7 



— Post-dorsal section. 

 — Minor lines of growth. 

 — Rest lines of growth. 

 — Lunule. 

 — Beak or umbone. 



Post-dorsal truncation. 

 Broken rays. 



Fig. 5b. — Internal view of right valve of P. securis. cf Same reduction 

 as in Fig. 5a. 



Fig. 6a. — External view of left valve of P. securis. Female shell reduced 

 one-fourth from natural adult size. Same age as adult. Comparison to male 

 shell of Fig. 5a. denoted by dotted outline. 



Fig. 6b. — Internal structures of right valve of P. securis. 9 . Same 

 reduction as in Fig. 6a. 



Ventral margin. 8. — Beak cavity. 



— Epidermis. 



— Mantle line. 



— Vein marks. 



— Ant. retractor mus. scar. 



— Ant. adductor mus. scar. 



— Cardinal tooth. 



— Interdentum. 



—Beak. 



— Hinge ligament. 



— Hinge tooth. 



— Protractor mus. scar. 



— Post add. mus. scar. 



