NO. 2 ROST: THE FAMILY ARCIDAE 179 



Family Arcidae 



Subfamily Arcinae 



Genus ARGA Linne 1758 



Area Linne, Systema naturae, ed. 10, p. 693. 



Type of genus: Area noae Linne 1758 (by action of the International 



Commission on Zoological Nomenclature, Oct. 5, 1944. Opinion 



189) 



Subgenus ARGA s. s. 



Area (Area) paeifiea (Sowerby) 1833 



Byssoarea paeifiea Sowerby, Proc. Zool. Soc. London, 1833, p. 17. 

 Fig.: Maury, 1922, PI. 1, fig. 15; Reinhart, 1943, PL 14, figs. 3, 4. 

 Anatomy: Heath, 1941. 

 Type loe.: Santa Elena, Ecuador; 6-18 fms. 

 Holotype: British Museum? 



Remarks: This species is extremely variable in form and shell propor- 

 tions. The posterior expansion is sometimes not very pronounced. Usu- 

 ally, the projection of the posteroventral expansion reaches behind the 

 posterodorsal one. Several authors state that A. {Area) paeifiea is more 

 expanded posteriorly than is the closely related A. {Area) zebra (Swain- 

 son) (= oeeidentalis Philippi) from the Caribbean. This character, 

 however, is not constant, as a specimen of A. {Area) zebra may some- 

 times be more expanded posteriorly than a specimen of A. {Area) 

 paeifiea of the same size. It seems difficult to find a really good char- 

 acter to distinguish the two species. 



The maximum height of the shell usually has to be measured per- 

 pendicular to the posterior part of the shell. Some specimens, however, 

 may have the maximum height perpendicular to the umbo. 



The anatomy of this species has been studied by Heath (1941), so 

 only a few observations will be mentioned here. The mantle margin is 

 furnished with eye-spots except for the part where the byssus emerges. 

 The margin is also pigmented with brown spots, and so are the ventral 

 part of the foot and the extreme posterior part of the gills. 



The largest specimen in this material measures 128 mm in length, 

 72 mm in height, and 80.5 mm in diameter. It is from the Gulf of 

 California and was dead when dredged. 



