PANAMIC-PACIFIC PELECYPODA 61 



gently arcuate; the posterior slope straight, the opposed margins of the 

 valves slightly pouting in the middle of a longitudinally striate, depressed 

 rostral area bounded on each side by a rounded angle; these terminate at 

 the slightly gaping end. A shallow, flattened ray of variable strength extends 

 from the beak to the anterior basal margin; the edge in front of this ray is 

 more or less crested by the concentric sculpture. Middle of valve sculptured 

 with strong concentric ripples but which fade out or become obsolete leaving 

 a smooth ray in front of the posterior rostral angle. 



Fresh specimens have a pale or subtranslucent straw-color and a thin 

 periostracum; such specimens usually show faint radial streaks, seen best 

 on the posterior side. Dead valves are porcellaneous white or yellow and 

 show no radial markings. The anterior ray is strong in most shells. The 

 sculpture is variable, typically with strong, rounded concentric riblets with 

 wide interspaces and this sculpture covers most of the disk except for a 

 narrow band below the rostral carina. As noted by Dall, an occasional speci- 

 men may occur which is smooth over most of the surface. 



Range — Lower California to Ecuador. Mexico: Lower California; Guay- 

 mas. Costa Rica: Port Parker (Hertlein and Strong). Panama: Bay (Dall). 

 Ecuador: Off Esmeraldas. 



Nucniana (Saccella) impar (Pilsbry and Lowe) Plate 2, figure 6 



Leda impar Pilsbry and Lowe, 1932, Proc. Acad. Nat. Sci. Philadelphia, vol. 84, pp. 



106, 107, pi. 17, figs. 3-6 "Guaymas". 

 Nuculana (Saccella) impar (Pilsbry and Lowe), Hertlein and Strong, 1940, Zoologica, 



vol. 25, pt. 4, p. 396, pi. 2, fig. 6. 



The shell is of medium size (length about 10 mm.), narrowly elliptical, 

 inequilateral, the beak located a little in front of the middle, the disk weakly 

 convex to depressed. Anterior side higher, somewhat elongated, its margin 

 obliquely rounded; the posterior side is somewhat longer, rostrate, concave 

 above, sharply pointed at the end and with a smaller point below, its mar- 

 gin, therefore, appearing somewhat bicarinate. The posterior-dorsal area is 

 narrowly depressed and strongly sculptured longitudinally, bordered by a 

 strongly noded rostral cord. Below the rostral cord, there is a narrow, de- 

 pressed zone across which the concentric riblets of the sculpture extend with 

 diminished strength. Surface sculpture consists of 10 or more strong, nar- 

 row, concentric riblets between wide, flattened interspaces; these concen- 

 trics are heaviest and most widely spaced over the middle of the disk, closer 

 together on the surface of the umbone and near the ventral margin. Hinge 

 with the anterior and posterior set of teeth of nearly equal length, the 

 resilifer between them quite small; there are 14 teeth in the posterior row, 

 all strong and nearly equal in size, except the first two on the inner side; 

 there are 18 teeth in the anterior row, the first eight at the inner end being 

 much smaller, the others increase rapidly in size towards the front. Pallial 

 line and sinus indistinct. The lunule is a small, narrow, depressed area, 

 transversely marked as if by the images of the teeth below. The rostral 

 carina is flatly noded by the ends of the concentric riblets and where these 

 are far apart, there may be one or two subsidiary nodes. Ventral margin 

 finely crenulate except at the extreme posterior end. 



Length 10.2 mm., height 5 mm., semidiameter 2mm. (a left valve 

 USNM). Off lower California, sta. 2822 in 21 fathoms U. S. Fish Com. 



Length 12 mm., height 6.2 mm., diameter 4.6 mm. (Pilsbry and Lowe). 



