146 AXEL A. CWLSSON 



and linear, covered by the ligament of which the resilium is developed as a 

 narrow wedge for about half its length. Hinge with one or two, small, 

 cardinal-like knobs under the beak and a set of more distant lateral laminae 

 and sockets. Surface of shell usually dark in color and rayed. 



Pterla sterna (Gould) Plate 18, figure 4 



Avicula sterna Gould, 1851, Proc. Boston Soc. Nat. Hist, vol. 4, p. 93. (Mazatlan).— 



Gould, 1853, Jour. Boston Soc. Nat. Hist., vol. 6, p. 404, pi. 16, fig. 7. 

 Avicula peruviana Reeve, 1857, Conch. Icon., vol. 10, Avicula, pi. 14, fig. 53 (Peru). 

 Pteria peruviana (Reeve), Dall, 1909, Proc. U. S. Nat. Museum, vol. 37, No. 1704, pp. 



150, 255, pi. 28, fig. 1.— Maxwell Smith, 1944, Panamic Marine Shells, p. 50, 



fig. 675. 

 Pteria sterna (Gould), Maxwell Smith, 1944, op. cit., p. 51, fig. 673.— Hertlein and 



Strong, 1943, Zoologica, vol. 28, pt. 3, No. 19, p. 164. 



Shell variable in shape, the axis inclined, the valves quite convex. 

 Young shells are usually much longer than high, the posterior wing narrow 

 and elongate. Larger and older shells, especially from Peru, become nearly 

 as high as long and the posterior wing is correspondingly smaller. Ground 

 color of worn shells is usually a dark purple, uniform or with narrow rays 

 of a lighter shade, the surface in unworn shell rough and coarse formed by 

 close-set radial spines or scales which near the ventral margin may form 

 large, overlapping, flattened spines as in Pinctada mazatlanica. 



Length 93 mm.; height 88 mm.; diameter 43 mm. Shell with short 

 posterior wing. Bayovar, Peru. 



Length 92 mm.; height 46 mm.; diameter 30 mm. Form with a long 

 posterior wing. Lobitos, Peru. 



As suggested by Hertlein and Strong, P. sterna and P. ■peruviana are 

 probably forms of a single species although studies in the future may show 

 that the southern or Peruvian shell should be accorded subspecific standing. 

 This species is often common along the shores of northwestern Peru where 

 it has occasionally been fished for pearls and large piles of its discarded 

 shell are to be seen at many places. The shell becomes large (100 mm. or 

 more), nearly as high as long, and the posterior wing is not greatly ex- 

 tended; smaller or immature specimens have the greatly prolonged, narrow 

 wing of typical P. sterna. 



Range — California to northern Peru. Ecuador: Manta; Santa Elena. 

 Peru: Mancora; Cabo Blanco; Lobitos; Negritos; Paita; Bayovar. 



Pteria beillana, new species Plate 18, figures 5-5c 



Shell relatively small (length 37 mm.), inequivalve, the left valve 

 being strongly convex, the right valve less so. The anterior ear is unusuallly 

 large, set-off rather sharply by a grooved line in the left valve and by a 

 deep, byssal sinus in the right. Dorsal margin straight, equal to the greatest 

 length of the shell, the small beaks elevated slightly above it a little in 

 front of the middle point. Hinge with an elongate, anterior lateral and 

 posterior lateral tooth in the right valve each fitting into a socket in the 

 left valve; the anterior tooth is quite close to the beak, the posterior about 

 midway along the dorsal margin. Resilium is quite long, attached to an 

 elongated scar entirely posterior of the beak, its length about half that 

 of the posterior-dorsal margin. Interior brilliantly pearly with a single, 

 large adductor scar placed on the posterior side of the shell cavity. Ex- 

 ternally, the color is a dull brown, smoothish on the umbones and with 

 radially frilled or scalloped concentric lamellae on the ventral sides and 

 on the ears; the outer shell layer forms a narrow band along the ventral 

 margin. 



