Bull. U. S. B. F., 1921-22. (Doc. 922.) 



PRet 



PRF- 



ARet 



AAd 



Fig. 139. — Photomicrograph of a byssus thread and end plate. X 10. 



Fig. 140. — Ventral view 1 f foot and retractor muscle. 



Fig. 141. — Side view of adductor and retractor muscles. 



Abbreviations.— .11./. anterior adductor muscle; ARet, anterior retractor muscles; By, byssus; ByP, byssus 

 pit; ByT, byssus thread ; /;/', cud plate; Ft, foot; PA J, posterior adductor muscle; PRB. posterior retractor muscles 

 of byssus; PRet, posterior retractor muscles; PRF, posterior retractor muscles of foot; VGr, ventral groove. 



