58 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 23 



only p. hybridum having ventral margin of right valve flexed. Auricles 

 varying from nearly equal to quite unequal, posterior being much smaller 

 in P. tigerinum ; byssal notch rather shallow to moderately deep. Right 

 valve either smooth, having minute concentric growth lines or ridges, or 

 with minute and obliquely divergent radial striae; P. tigerinum occa- 

 sionally also radially corrugated or with fine radial ridges, minute radial 

 striae always being present. Left valve with minute and obliquely diver- 

 gent radial striae and often with fine radial ridges which may be minute- 

 ly spinose; P. tigerinum occasionally also radially corrugated, always 

 with minute radial striae. 



Remarks: Polliolum was regarded as a synonym of Pseudamussium 

 by North (see Preface). After studying many specimens of the west 

 African hybridum (Gmelin) and the western Atlantic incomparabilis 

 Risso, tigerinum (Miiller) and striatum (Miiller), the present author 

 agrees, finding no basis for generic separation. Incidentally, tigerinum is 

 an unusually variable species, for on the basis of structural variations 

 certain specimens could be referred to either Pseudamussiu7n s. s., Pep- 

 lum, Aequipecten or Flexopecten. 



There has been considerable disagreement as to the first validation 

 of this genus, the identity of its type species, and the valid name for the 

 latter. North (1951, p. 231) discussed the problems, and in his un- 

 published Classification included a further discussion. The author is con- 

 vinced that North, after exhaustive analysis, arrived at the only logical 

 conclusions: the genus dates from the Adams,' the type species is the 

 west African shell, and its valid name is hybrida Gmelin. 



After studying all the original descriptions and figures cited by 

 North, the author searched the literature for other references to the 

 genus, designations of type, and various specific names involved in the 

 case. The evidence indicated North's conclusions were correct. 



A history of this unit and its type species follows, with comments on 

 most of the references. 



1. Lister (1685, tab. 173, fig. 10) gave this description of a shell: 

 "Pectenibus aequaliter auritis, Laevibus. Pecten laevis, variegatus, 

 admodum planus sive compressus." The figure was well-drawn, and 

 easily recognizable as the west African shell. [Lister pre-Linnean 

 and invalid.] 



2. Klein (1753, p. 134, pi. 9, fig. 31) reproduced Lister's figure, 

 named the shell Pseud-amusium, and gave a description based on 

 Lister's. [Klein also pre-Linnean and invalid.] 



