14 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 



This species differs in its sculpture from any of the recent species 

 ascribed to the coast, and from A. barbarensis Conr. (Pliocene foss.) by 

 its shorter, more elevated, and deeper form, as well as by details of 

 sculpture. 



Pecten expansus, n. 8. (7941). 



Shell large, thin, with the upper valve flatter than the lower one, both 

 with very slight convexity; outer surface of upper valves marked by 

 sixteen to twenty sharp, radiatiug ridges, but slightly elevated, and 

 whose sides shade off insensibly into the broad interspaces, which are 

 but slightly depressed; faint indications of ridges appear between the 

 principal ones. The entire surface is covered with fine, slightly raised, 

 sharp lamellae, which are waved in some places so regularly as to pro- 

 duce the appearance of a delicate reticulation, which, however, does 

 not really exist; angle of the umbo about 120°; ears finely sculptured, 

 like the rest of the surface, but with only faint indications of ridges, 

 sharply differentiated from the rest of the shell, very short, broad; 

 supra-foraminal ear with a sigmoid curve to the lateral margin ; mar- 

 gin of the other ear nearly straight; hinge-line straight; interior of the 

 valve smooth, except for faint depressions corresponding to the ridges; 

 peripheral margins not crenulated, even or nearly smooth. 



Lower valve with twenty-five or thirty dichotomous ribs, flattened 

 above, but not sharply differentiated from the interspaces, sculptured 

 with fine lines of growth or nearly smooth, with faint appearances of 

 radiating stria*. Peripheral margin somewhat crenulated by the ends 

 of the ribs; interior marked by shallow channels corresponding to the 

 ribs; ears rather small and distinctly but not strongly marked off from 

 the rest of the valve ; byssal notch rounded, moderately deep. Height 

 of shell, 335"""; breadth of shell, 140 mni ; breadth of hinge-line, G5 ,um j 

 thickness, 32 mm ; some specimens ouehalf larger. 



This shell is nearest P. yropatulus Conr. (caurinust of Gould) from 

 the Miocene of Oregon, but differs in all its details when compared. 

 The Miocene shell has a sharper umbonul angle, larger ears with straight 

 lateral margins, and strong and different sculpture; the ribs are not 

 dichotomous, and are much more sharply defined, while the margins are 

 strongly crenulated. It is possible that some of the indeterminate 

 nominal species of Conrad may have been based on this species, but the 

 wretched figures given by him seem to differ strongly so far as they 

 show any characters, while his descriptions are quite worthless, as usual. 



Pecten Stearnsii, n. h. (7942). 



Shell moderately large, thin, regular; elegantly radiately ribbed. 

 Upper valve flattened or even a little concave, with about twenty four 

 regularly rounded, vaulted, even ribs, separated by slightly wider chan- 

 nelled interspaces ; the whole surface covered with fine, sharp, conceu- 

 tric, regular lamellae, a little looped backward over the top of the ribs, 

 but showing no appearance of reticulation anywhere; ears small, nearly 



