PANAMIC-PACIFIC PELECYPODA U3 



12. Anterior end of shell full, its margin well rounded, and with small 

 normal crenulations only. The posterior adductor scar is quite large, 

 confined to the dorsal side. 



Genus Hormomya 



13. Anterior end of shell narrowed and pinched, its dysodont teeth large 

 and coarse. Posterior adductor scar circling the posterior end of the 

 valve on the inside. 



Genus ScolimytUus 



dabbb. Anterior margin turned in dorsally so as to lie closely under the 

 beaks and ends in a sharp, cutlike break, the ligament does not extend 

 forward of this point. 



14. Shell full, convex, fan-shaped, coarsely ribbed but with the surface 

 usually eroded. Color deep purple or black. 



Genus PerumytUus 

 db. Borers in rock or coral growth. 



15. Shell cylindrical, bullet-shape, the beaks terminal. Surface smooth or 

 encrusted but not corroded. 



Genus Lithophaga 



16. Shell elongated, irregular, the beaks near the middle, the umbones 

 and midzone often deeply corroded. 



Genus Adula 



17. Shell modioliform, sculpture with fine radials except in the midzone 

 which is smooth, the periostracum covered with bristles. 



Genus Gregariella 



Genus MTTILUS Linn6, 1758 



Type species by subsequent designation, Anton, 1839, Mytilus edulis 

 Linne. North Atlantic. 



Shell mytiliform or ovate- oblong, usually with a strong, oblique axis. 

 Beaks placed at the extreme anterior end are small, the posterior side 

 wide and rounded, convexly vaulted along the umbonal arch extending 

 from the beak to the posterior-ventral corner. Hinge with a series (about 

 four to six), small, dysodont teeth on the anterior margin or narrow plate 

 below the beak. Ligament subinternal, narrowly hnear in shape, within the 

 dorsal margin, and attached to a whitish chalky resilifer, the basal founda- 

 tion of which is finely pitted. Anterior adductor scar small, lying in the 

 cavity of the beak and connected with the pallial line which lies as a gradu- 

 ally widening band within and parallel to the ventral margin; posterior 

 adductor and retractor scar united and forming an irregular pipe-shaped 

 impression. Cold water. 



M. edulis is typically a North Atlantic species but various closely re- 

 lated forms or subspecies have been named from the Southern Hemisphere. 

 M. edulis Linne, Plate 12, figure 6. 



Mytilus arciformls (Ball) Plate 12, figures 4-4b 



Modiolus arciformis Dall, 1909, Proc. U. S. Nat. Museum, vol. 37, No. 1704, pp. 152, 



258, pi. 28, fig. 2 Huaquilla, Ecuador. 

 Volsella (Volsella) arciformis (Dall), Hertkin and Strong, 1946, Zoologica, vol. 31, 



pt. 2, p. 72, pi. 1, fig. 5 Monypenny, Nicaragua. 



