126 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 23 



var. /?) major, solidior, costis 50, 25'" alta, 23>4'" lata, SJ^"' 

 crassa. 



The coloring is varied ; some are almost colorless, others yellowish 

 with brown on the upper valve ; others have a few yellow or brown ribs 

 on a colorless background. [Translated from the German.] 



Additional description: Shell rather small and nearly transparent; 

 almost orbicular (slightly higher than long) ; adult specimens averaging 

 30 mm in height ; both valves moderately to rather deeply convex and 

 frequently slightly oblique; hinge line about % length of disk. Ribs of 

 both valves result of corrugations of disks and number ranging from 30 

 to 50; width and height irregularly varying and divarication frequent; 

 occasionally higher and narrower on left valve than on right. Auricles 

 very unequal, anterior nearly twice as long as posterior. Right valve 

 appearing completely smooth, but having very minute oblique striae 

 along submargins ; anterior auricle with fine concentric striae and 3 to 5 

 distinct rounded folds separated from disk by broad fasciole ; ctenolium 

 of 5 teeth ; posterior auricle with several faint ridges, very minute 

 horizontal striae and fine vertical growth ridges. Left valve with ribs 

 smooth but interspaces distinctly concentrically lamellose; anterior 

 auricle covered with fairly strong concentric striae and having 5 to 7 

 distinct radiating ridges ; posterior auricle with 4 or 5 faint ridges, 

 minute horizontal striae and stronger concentric striae. Coloring as 

 indicated by Philippi. 



Remarks'. This species is quite distinct; it resembles, in some respects, 

 only one other species of Pectinidae: Juxtamusium maldivensis (E. A. 

 Smith). 



In referring Pecten natans to Juxtamusium the author followed the 

 suggestion of F. K. North, whose unpublished classification is cited in the 

 preface to this paper. Neither Pseuda?nussiu7?i nor Chlamys are logical 

 choices; Leptopecten was considered, but the size and shape of the 

 auricles of Pecten natans prohibit its inclusion in that subgenus. For the 

 present, Juxtamusium seems most appropriate. 



Geographical range, eastern Pacific: southern portion of Magallanes 

 Province, Chile, and western portion of Tierra del Fuego Territory 

 (Chile) ; western Atlantic: eastern portion of Tierro del Fuego Terri- 

 tory ( Argentina ) . 



Geochronological range: Available records indicate Recent only. 



Bathy metric range: No records located; probably only in shallow 

 water. 



