448 Stimpson on the Crustacea and Echinodermata 
Among the fresh-water Crustacea we may notice the 
fact, first observed by Dana* in Astacus leniusculus, that 
the Cray-fish of the rivers running into the Pacific have 
branchiz on the fifth pair of legs, and, like those of Europe, 
are classed among the true Astact. Agassiz} saw the same 
thing in A. Gambelii, and I have found it to be invariably 
the case in the species of this region, among which there 
are several not hitherto described in the Museum of the 
Smithsonian Institution. Our eastern Cray-fish, on the 
contrary, all belong to the genus Cambarus, having no 
branchiz on the legs of the fifth pair,—a singular instance 
of the coincidence of peculiarities of structure with those of 
geographical distribution. Emrcusoxw f does indeed describe 
two species of Cambarus from Southern Mexico, but we 
have reason to suppose that these belong rather to the east- 
ern slope of the Rocky Mountains. 
In the preparation of the following paper I have used 
every means in my power to identify the species described 
by previous authors, and have done this by actual compari- 
son of specimens wherever it was possible. Through te 
kindness of Dr. Brivezs I was enabled, during a short visit 
to Philadelphia for that purpose, to examine the ty ical 
specimens of RANDaLU's species, and those of De Sa | 
To Professor Dana I am indebted for much assistan 
for the use of the few specimens of his types which *. 
particularly desired for comparison,—the admirable exact- 
ness of his figures and descriptions rendering amy furt 
means of identification in most cases unnecessary- : 
Full-descriptions will be here given of the new species 
only, but notes are appended to several already know 
including remarks on characters which have been c 
looked by previous authors. Enough is given 1n most 
instances to enable the reader to determine any know? 
California or Oregon species. 
* U. S. Exploring Expedition, Crust. i. 524. 
T Proc. Acad. Nat. Sci., Philad. vi. 375. 
1 Archiv für Naturgeschichte, 1846, i. 99. 
