234 AXEL A. OLSSON 



Hysella compressa (Dall) Plate 35, figure 10 



Rochefortia compressa Dall, 1913, Proc. U. S. Nat. Museum, vol. 45, No. 2002, p. 596. 

 Head of Concepcion Bay, Gulf of California as given on label of type USNM 

 214445.— Dall, 1921, Bull. U. S. Nat. Museum, No. 112, p. 37, pi. 3, fig. 1. 



Shell small, compressed to slightly convex, rounded-quadrate to sub- 

 ovate, the anterior side longer and widely rounded at the end, the shorter 

 posterior side with its dorsal margin descending rather sharply to a rounded 

 or subtruncated end. Hinge has a wide V-shaped notch in the middle with 

 the beak at its tip, and directly under it in the cavity lies the scar of the 

 resilium. In the right valve, the median notch is bordered by lamellar teeth 

 on each side, and above each of these a grooved socket for the reception 

 of the thickened, bevelled edge of the opposite valve. Surface white, more 

 or less roughened by fine to coarse lines of growth. Adductor scars subequal, 

 connected by an entire, ribbon-like pallial line. 



Length 6 mm., height 4.6 mm. Zorritos, Peru. 



Range — ^Alaska to northern Peru. Peru: Zorritos. 

 Mysella negrritensis, new species Plate 35, figure 8 



The shell is relatively large for the genus, white, high, ovate, depressed 

 to slightly convex. The posterior side is short, its margin descending sharply 

 to assume at its end, a slightly rounded or truncated shape. The hinge of 

 the left valve has the usual deep umbonal notch carrying the resilifer in 

 the cavity under the beak and it is bordered by the bevelled edges of the 

 margin on each side which fit snugly into the grooved sockets of the opposite 

 valve. The adductor scars are of nearly equal size, the connecting pallial line 

 entire. Surface marked with rough, irregular lines of growth. 



Length 10.3 mm., height 8.5 mm., diameter 2 mm. (a left valve). 

 Negritos, Peru. Holotype, ANSP 218889. 



The smaller shells are generally smoother than the large ones, the perio- 

 stracum thin and of light brown color. This species is fossil in the Pliocene 

 of Ecuador. 



Range — Peru. Peru: Negritos. 



Genus ALIGENA H. C. Lea, 1843 



Type species by subsequent designation, Dall, 1900, Aligena striata 

 H. C. Lea {=zA. aequata Conrad). 



Shell small, rounded-ovate to oblong trigonal, the anterior side gen- 

 erally longer, inflated, thin, white, with full umbones and small, prosogyrate 

 beaks. Surface of the valves evenly inflated or with a more or less strong, 

 constriction over the middle. Hinge weak, edentulous, or with a single, 

 small, anterior tooth under the beak and behind it a wide, open gap or 

 notch. Ligament internal, the resilifer elongated and placed in the margin 

 of the hinge notch. 



Aligena cokerl Dall Plate 33, figures 6, 6a, 6b 



Aligena cokeri Dall, 1909, Proc. U. S. Nat. Museum, vol. 37, No. 1704, pp. 155, 264, 

 pi. 28, figs. 5, 6. Attached to worm tubes thrown upon the beach of the lagoon 

 at Capon, Peru.— T. Burch, 1941, Nautilus, vol. 55, No, 2, p. 48.— Hertlein and 

 Strong, 1947, Zoologica, vol. 31, pt. 4, p. 134. 



The shell is quite small, white, thin, tumid and more less strongly 

 constricted in the middle, the shape usually rounded ovate but varying 



