CLASS PELECYPODA 
83 
Range. San Pedro, California, to Guayaquil, Ecuador. In the Pleis¬ 
tocene at San Pedro and San Diego, California. 
SECTION HALISTREPTA Dali, 1904. 
Periploma sulcata Dali, 1904. 
Plate 40, fig. 6. 
The Nautilus , 17:122. Bull . Comp. Zool, 43; pi. 15, fig. 10. 
Shell rotund, white, with the left valve flatter, thin, sculptured with 
numerous, close-set, irregularly concentric, more or less interrupted, low 
ridges, separated by sub-equal, shallow interspaces; the surface is also 
microscopically shagreened, and there is a low rib extending from the 
beak to the lower margin of the ill-defined rostrum and an ill-defined 
furrow radiating from the beak toward the anterior base, in the right 
valve; beaks low, distinctly fissured; anterior dorsal hinge line rounded, 
posterior ditto, shorter, nearly rectilinear, forming with the elevated rib 
a sub-triangular space which is free from the undulations which cover 
the rest of the shell; interior shining, hardly nacreous, the muscular im¬ 
pressions very small, the pallial line obscure; chondrophores prominent, 
spoon-shaped, extending obliquely forward, and with their connecting 
resilium sustaining a proportionately large triangular lithodesma; the 
chondrophores are supported behind by well-developed clavicular props, 
which are inserted posteriorly on the surface of the valve below the 
linear hinge line. Length, 32; height of right valve, 6, and of left valve, 
4 mm. (Dali.) 
Type in U. S. N. M. Type locality, Terminal Island, San Pedro, 
California. 
Range. Known only from the type locality. 
Family THRACIDyE. 
Genus THRACIA (Leach) Blainville, 1824. 
Shell concentrically striated, with more or less fine, superficial granula¬ 
tion and a very delicate periostracum, sub-rostrate, slightly gaping behind; 
slightly inequivalve, the right valve larger; the beaks in contact and usually 
perforated by friction on each other, the hinge plate fissured below them 
and edentulous; the ligament external, the resilium more or less sunken 
and, in most cases, with a short, transverse lithodesma in front of it, 
occupying the fissure in the hinge plate; pallial line with a moderate sinus, 
margins of the valves entire; the nymphs in the typical forms do not 
project greatly from the hinge margin ventrally and are more or less 
elongated; the shell is destitute of nacre. 
