NO. 2 rost: the family arcidae 189 



Another group of Acars, containing species which never reach the 

 size of Acar gradata, at its northern limit is named Acar bailyi Bartsch 

 1931 {==pernoides Strong 1932, perhaps not Carpenter 1856b), with 

 type locality Balboa, California; and at its southern limit is named Acar 

 pusilla (Sowerby) 1833, with type locality Iquique, northern Chile. 

 The range of A. bailyi is given by Reinhart (1943) as from Topanga 

 Beach (near Santa Monica), California, to Geronimo Island, Gulf of 

 California (H. N. Lowe collection). 



In the material at hand, specimens very close to Acar bailyi were 

 found on the Mexican islands, Socorro, Clarion, Isabel, etc., and on the 

 Islas Secas, Panama, as well as on the Galapagos Islands. As it has been 

 impossible for me to find distinguishing characters, they are here treated 

 under A. bailyi. 



Garth (1946) discussed species of Brachyura which have a similar 

 distribution: species which occur in Baja California and the Gulf of 

 California and reach the Galapagos Islands via the Mexican islands, 

 some of them also occurring in the Bahia de Panama. He also discussed 

 the causes of such a distribution. 



One sample in the material from Ecuador seems to represent speci- 

 mens of Acar pusilla (Sowerby), a little known species which is very 

 close to A. bailyi. 



Acar gradata (Broderip and Sowerby) 1829 

 PI. 12, figs. 11-12 



Area gradata Broderip and Sowerby, Zool. Jour. London, vol. 4, 1829, 



pp. 365-366. 

 Syn.: Area (Byssoarca) pholadiformis C. B. Adams 1852 (not A. 

 pholadiformis Orbigny 1844). 



Barbatia {Acar) reticulata Dall 1898, non Gmelin 1791. 



Acar pana?nensis Bartsch 1931. 

 Fig.: Maury, 1922, PI. 2, figs. 4, 6, 9; Bartsch, 1931; Reinhart, 1939, 



PI. 3, figs, la, lb. (Holotype) 

 Anatomy: Heath, 1941. 

 Type loc: Mazatlan, Mexico. 



Holotype: British Museum. Zoological Dept. no. 58.5.12-100. 

 Remarks: As discussed under genus Acar, this species may be coarsely 

 or finely sculptured. The finely sculptured form is in the minority in the 

 present material. Some samples contain both fine and coarse specimens. 

 Judging from the material at hand, the finely sculptured form is most 

 common at the extremities of the range of the species, namely, in Baja 

 California and Ecuador, whatever the cause may be. 



