of the Pacific Shores of North America. 907 
great velocity among the stones or debris of shores after the 
retreat of the tide. They are never found elsewhere than 
in elose proximity with water, which may be either salt, 
brackish, or fresh. Z. occidentalis was originally discovered 
on the banks of the Sacramento River, by Dr. Pickering of 
the Exploring Expedition, and has since been found in 
various parts of California. 
LYGIA DILATATA. Stimpson. 
PI. X XIELE.8. 
Lygia dilatata, SriwPsox ; Proc. Bost. Soc. Nat. Hist. vi. 88. 
Body variable in its proportions, but usually very broad; 
the proportion of the breadth to the length being often 
:15. Surface granulated. Margins of the articulations 
raised or thickened, and smooth. Head with a transverse 
tidge between the eyes, interrupted at the middle. External 
antenne not very slender, reaching the sixth thoracic seg- 
ment; flagellum consisting of fourteen scarcely oblong 
joints. Caudal appendages, very short, generally not more 
than one fifth the length of the body, often even shorter ; 
basal joint or peduncle as broad as long, with a sharply 
Produced angle exterior to the insertion of the stylets, the 
ner one of which is provided with a terminal bristle as in 
L. occidentalis. Color blackish. 
9ung specimens are much less broad than the adults, 
35 the breadth increases with growth much faster than the 
length. It is at first difficult to conceive how they can 
belong to the same species, but a careful examination of 
Specimens of all ages shows this to be the, case. The di- 
Mensions of two specimens are as follows :— 
Adult, length,....1.42 breadth,....0.96 inch. 
Young, “ 0. « gs ^ 
Found in considerable numbers in the summer of 1856, 
at Fort Steilacoom, Puget Sound, by Dr. George Suckley, 
* gentleman to whose assiduous and successful researches 
field of natural science we are indebted for many 
