188 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 20 



height, and 21.3 mm in diameter, has seven grooves, three of them 

 reaching before the umbo. 



The periostracum consists of fine hairs connected with a foliaceous 

 membrane. The hairs reach beyond the edge of the membrane, forming 

 serrations. The mantle margins are unpigmented. The abdominal sense 

 organs resemble those found by Heath (1941) in "Barbatia harhata." 

 The byssus is well developed. 



The H/L ratio varies between 50 % and 75%, the D/L ratio be- 

 tween 43% and 57%. The largest specimen in this collection measures 

 38.1 mm in length, 19 mm in height, and 15.5 mm in diameter. It is 

 from Puerto Utria, Colombia, shore (232-34). 



Occurrence: B. (Fugleria) illota is usually taken in the intertidal zone 

 attached to rocks. The species has also been dredged in 34 and 40 fms. 

 Distribution: Isla Angel de la Guarda, Gulf of California, to Lobitos, 

 Peru (the Peru locality reported by Olsson, 1924). 



Genus AGAR Gray 1857 



Acer Gray, Ann. and Mag. Nat. Hist., sen 2, vol. 19, 1857, p. 369. 

 Type of genus: Area divaricate (Sowerby) 1833. (Subsequent designa- 

 tion by Stoliczka, 1871). 

 Remarks: As pointed out by Bartsch ( 1931), Acar is such a distinct unit 

 of the subfamily Arcinae that it ought to be given generic rank. In the 

 Acar-grou^, however, each species is not distinctly circumscribed, and the 

 variability of the species makes it a very puzzling and interesting genus. 

 The present material contains approximately 80 samples of Acar, rang- 

 ing from southern California to Peru and the Galapagos Islands. 



One species group, known as Acar gradata (Broderip and Sowerby) 

 1829, reaches a relatively large size and occurs in two forms, one coarse- 

 ly sculptured {panamensis Bartsch 1931) and one finely sculptured 

 {gradata Bartsch 1931). Reinhart (1939, PI. 3, figs, la, lb) shows 

 that the holotype is the coarsely sculptured form, which makes panamen- 

 sis Bartsch a synonym of gradata, leaving the finely sculptured form 

 without a separate name. Reinhart (1939, 1943) considers it unneces- 

 sary to treat them as different varieties, as it sometimes may be difficult 

 to decide whether a specimen belongs to the finely or coarsely sculptured 

 form, and as the distribution of the two forms almost coincides. The 

 two forms are not treated separately in the present paper.* 



*After the present manuscript was finished, Dr. S. Stillman Berry (Leaflets 

 of Malacology, vol. 1, no. 12, July 1, 1954) gave this form a new name, Barbatia 

 {Acar) rostae, considering it to be a valid species with good characters separating 

 it from A. gradata. 



