86 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 23 



eight unequal, elevated, angular ribs, with numerous erect, slightly 

 curved, sharp spines, and several smaller spinose ribs in the interstices. 

 The lower valve has 22 more nearly equal ribs with more numerous 

 and smaller spines on the angles. The only specimen we have seen is in 

 the collection of the Rev. F. J. Stainforth. 



Additional description : Shell averaging about 64 millimeters in 

 altitude, slightly shorter than high, inequivalve, equilateral (except for 

 ears), compressed and with serrate margins; base evenly rounded be- 

 low; sides slightly concave above. Right valve with 9 pairs of narrow, 

 elevated, spiny ribs, along the sides of which are a pair of smaller spiny 

 riblets; major interspaces much wider than ribs, concave-bottomed and 

 ornamented by 1 or 3 thread-like, spiny, intercalary riblets ; whole sur- 

 face sculptured by microscopic imbricating lines, of which the spines on 

 the ribs and riblets are only modifications; hinge line equal to one-half 

 length of disk; ears unequal, the anterior being about two and one-half 

 times the length of the posterior; anterior ear with 7 or 8 prominently 

 spiny radials and minor riblets, and imbricating incremental lines ; byssal 

 notch deep and almost as wide as ear ; posterior ear small, and sculptured 

 by fine, spiny radials and imbricating incremental lines. Left valve with 

 about 9 narrow, very prominently spiny ribs ; interspaces wide, concave- 

 bottomed, and ornamented by a single prominent, central, imbricated 

 riblet, on each side of which are minor thread-like riblets; whole surface 

 of disk with fine incremental sculpture as in right valve ; ears similar to 

 those of right valve except that the anterior has more ribs and riblets 

 and no byssal notch. Hinge with almost obsolete cardinal crura. Color, 

 golden yellow to pink, the left valve being the darker. (Arnold, 1906, 

 p. 109.) 



Additional descriptive notes: Juvenile specimens up to about 15 

 mm in height are more elongated. The color range includes various 

 shades of red, pink, orange and yellow. The commensal gastropod 

 Capulus calif orniciis Dall is occasionally attached to the anterior side of 

 the umbo of the right valve, as in Pecten diegensis Dall, and lives only 

 on these two species, never alone or on any other mollusca. 



Young specimens of 15 mm or less in height resemble the young of 

 Hinnites multirugosus (Gale), but those of the latter can be distin- 

 guished by the following differences in structure: the disk equilateral (or 

 nearly so), the ribs of the right valve divaricating, the posterior auricles 

 larger and squared, the interspaces of both valves concentrically lamel- 

 lose. 



Remarks: Bavay's diagnosis of Pecten denticulatus Adams & Reeve 

 (Borneo; Sulphur expedition) as a juvenile Chlamys hastata was un- 



