of the Pacific Shores of North America. 507 



great velocity among the stones or debris of shores after the 

 retreat of the tide. They are never found elsewhere than 

 in close proximity with water, which may be either salt, 

 brackish, or fresh. L. 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. XXII. f. 8. 



Lygia dilatata, Stimpson ; 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 

 1:1.5. Surface granulated. Margins of the articulations 

 raised or thickened, and smooth. Head with a transverse 

 ridge between the eyes, interrupted at the middle. External 

 antennae 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 

 inner one of which is provided with a terminal bristle as in 

 L. occidentalis. Color blackish. 



Young specimens are much less broad than the adults, 

 as 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, " 0.98 " 0.45 " 



Found in considerable numbers in the summer of 1856, 

 at Fort Steilacoom, Puget Sound, by Dr. George Suckley, 

 a gentleman to whose assiduous and successful researches 

 in the field of natural science we are indebted for many 



