NO. 2 ROST: THE FAMILY ARCIDAE 191 



localities. As specimens of A car pusilla (Sowerby) from Ecuador to 

 Chile, and these specimens from southern California are very like each 

 other, it is possible that we have to do with a more widely distributed 

 species which occurs in a larger form in the intermediate area, especially 

 on the islands, and which therefore could be named Acar pusilla forma 

 insularis. It is sometimes difficult to separate it from young specimens of 

 Acar gradata. A. bailyi has distinctly striated teeth and the same ar- 

 rangement of pigmented eye-spots as has A. gradata. 



Heath ( 1941 ) has made an interesting survey of the anatomy of this 

 species {"Barbatia {Acar) pernoides" from off Baja California). He 

 shows that in several anatomical structures, this species is different not 

 only from other species of the genus Acar {gradata and "reticulata"), 

 but from all other Areas he has studied. 



The largest specimen of Acar bailyi in this material, from Isla de 

 Clarion (140-34), measures 13.3 mm in length, 8 mm in height, and 7.8 

 mm in diameter. Many specimens have a very large diameter, often 

 measuring more than the height. 



Occurrence: Intertidal on rocky shores, and also in shallow water on 

 coral reefs, where it seems to thrive very well. One sample with ten 

 specimens is labelled 274-34, a station off Navidad Head, Mexico, which 

 according to the station list is from 50 fms, with bottom of mud and 

 sand. The record seems questionable, as all other samples with living 

 specimens are either intertidal or from shallow water. Previously, it 

 was believed to be a more northern species (see Strong, 1932, pp. 27-29; 

 Reinhart, 1939, p. 42). Reinhart (1943, p. 35) gives its range from 

 Topanga Beach, near Santa Monica, California, to Geronimo Island, 

 Gulf of California. 



Distribution: Santa Monica, California, to Panama; the Galapagos 

 Islands. 



Acar pusilla (Sowerby) 1833 

 PI. 12, fig. 13 



Byssoarca pusilla Sowerby, Proc. Zool. Soc. London, 1833, pp. 18-19. 

 Syn.: fAcar gradata (finely sculptured form) auct. non Broderip and 



Sowerby 1829. 

 Fig.: Reinhart, 1939, PI. 3, figs. 2a, 2b. (Holotype) 

 Type loc. : Iquique, Chile ; attached to stones at low water. 

 Holotype: British Museum. Zool. Dept. No. 58.5.12-100. 

 Remarks: One sample from Isla La Plata, Ecuador, containing seven 

 specimens, represents this species. An illustration is given for comparison 



