162 AXEL A. OLSSON 



Generally similar to L. subnodosus but with more numerous ribs (13 

 to 15), the nodes usually smaller or absent and with less heavily sculptured 

 ears. 



Range — Ecuador and the Galapagos Islands. 



Genus AEQUIPECTEN Fischer, 1887 

 Type species by monotypy, Chlamys opercularis Linne. Recent, Europe. 



Shell small or large, subequivalve, the left valve somewhat more 

 inflated than the right. Sculpture formed by fairly numerous, rounded 

 or flat-topped ribs between fairly wide, shallow or deep, narrow inter- 

 spaces, the surface of both ribs and interspaces overrun by subsidiary 

 longitudinals or concentrics. Ears subequal, the right anterior cut into by a 

 byssal notch. Resilifer or chondrophore is fairly wide and deep, flanked on 

 each side by a cardinal crura. 



Subgenus PLAGIOCTENIUM Dall, 1898 

 Type species by original designation, Pecten ventricosus Sowerby 

 (=zcircidaris Sowerby). 



Valves subequal, convex, with strong ribs without radial striation but 

 with a minute sculpture of raised, concentric threads or lamellae looped 

 across the ribs and interspaces, widely spaced on the umbones, more closely 

 crowded ventrally. Submargins impressed, smoothish. Auricles subequal, 

 the right anterior ear with a deep, byssal notch. A set of strong, crural 

 teeth in the right valve fitting into sockets in the left, minutely, vertically 

 striated. 



Two species. 



I. Shell large, often 100 mm. or more. Both valves with moderate con- 

 vexity, Peruvian faunal area. 



A. purpuratus 



II. Shell of medium size, 60 mm. or less. Both valves are strongly convex. 

 Entire Panamic-Pacific faunal area. 



A. circularis 



Aeqnipecten (Plagrloctenlimi) pnrpni^tns (Lamarck) Plate 19, figures 1-lb 



Pecten purpuratus Lamarck, 1819, Hist. Anim. s. Vert, vol. 6, pt. 1, p. 166. — Sowerby, 

 1843, Tiles. Concli. vol. 1, p. 52, pi. 15, fig. 113; pi. 16. figs. 123-125.— Reeve, 

 1852, Conch. Icon., vol. 8, Pecten, pi. 5, fig. 25.— Dall, 1910, Proc. U. S. Nat. 

 Museum, No. 37, pp. 149, 256, pi. 26. figs. 5, 6. 



Shell of medium or large size (up to about 130 mm.), circular, sub- 

 equivalve and moderately inflated, the right valve generally a little more 

 convex and with a higher umbone. On typical specimens, ribs number from 

 21 to 23 merging into the smaller riblets on the sides. The ribs of the right 

 valve have a flattened, rectangular section, much wider than their deeply 

 grooved interspaces. Ventially, the ribs appear to widen, their corner 

 angles bevelled off and crossed by squamosa, elevated threads. Ribs on 

 the left valve are generally more triangular in section, their summits if 

 flattened, narrower and their sides more deeply bevelled. Interspaces and 

 sides of the ribs are covered by evenly spaced, squamosa cross threads. 

 Large specimens may have one or more interstitial riblets in the interspaces. 

 The color is generally some shade of violet or purple, plain or mottled, the 

 interior white or with brownish discoloration. 



