192 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 20 



with Reinhart's figure of the type. The species is easily recognized by 

 the lacking or faintly developed radiating sculpture on the median part 

 of the shell. It is small, the largest specimen at hand having a length of 

 8 mm, the type of about 11 mm. The teeth are striated as in Acar 

 gradata. No specimen is preserved in alcohol. It can be seen from the 

 dried-out animals, however, that Acar pusilla has a strong byssus and is 

 furnished with the same arrangement of eye-spots as A. gradata. 



Dall (1910) and others refer to Reeve, 1844, PI. 16, fig. 112, as this 

 species. This is wrong, as shown by Maury (1922, pp. 19-20). Acar 

 pusilla is not figured in Conchologia Iconica, Reeve's species being 

 similar to the finely sculptured form of A. gradata. Carpenter (1856a, 

 p. 142), who apparently had seen the type of A. pusilla, indicates that 

 the pusilla of Orbigny might be a dwarf variety of A. gradata. However, 

 as Orbigny (1846, p. 633) reports A. pusilla from Peru, and from 

 Arica and Cobija, Chile (22° 30' S. lat.), it is questionable if his 

 records were A. gradata. Orbigny also refers to Reeve's species 112. 

 Occurrence: Intertidal on rocky shores. The only sample in the present 

 collection is from Isla La Plata, Ecuador. 



Distribution: Dall (1910) gives its range from the "coast of Ecuador, 

 and south to S. lat. 23° 37' " (Isla Blanca, Chile). 



Genus ARCOPSIS von Koenen 1885 



Arcopsis von Koenen, Abh. Gesell. der Wiss. Gottingen. Physik. KL, 



Bd. 32, Tl. 2, 1885, p. 86. 

 Type of genus: Area limopsis von Koenen 1885 (subsequent designation 



by Reinhart, 1935). 

 Remarks: Species belonging to Arcopsis have a rhomboidal, transversely 

 striated ligament. The genus is related to the fossil genus Striarca Con- 

 rad 1862. MacNeil (1938) raises Striarca to subfamily rank and refers 

 it to the family Noetiidae, which in turn he refers to the superfamily 

 Glycymeracea, Genus Arcopsis is placed in the subfamily Striarcinae. 

 This classification is not followed in the present paper. (Cfr. discussion 

 by Reinhart, 1943, p. 5 and pp. 76-77.) 



Arcopsis solida (Sowerby) 1833 



PI. 12, fig. 10; PI. 13, fig. 16 



Byssoarca solida Sowerby, Proc. Zool. Soc. London, 1833, p. 18. 

 Fig.: Maury, 1922, PI. 2, figs. 7, 12. 

 Anatomy: Heath, 1941. 



