ASTACUS. 125 
ABSA CUlSe 
Iy the genus Astacus the last thoracic somite bears a gill (pleuro- 
branchia) on each side, the full number of gills being thirty-six (eighteen 
in each branchial chamber). There are besides two or three rudimentary 
gills on each side of the body. The hindmost podobranchia is provided 
with a plaited, bilobed lamina, like those in front. The arrangement of the 
gills is expressed in the following formula : — 
Somire. PODOBRANCULE. ARTIROBRANCHLE. PLEUROBRANCHILA. 
KiGie ~~ Posterior. 
Wiles. «..0'Cep.) Orem Oe: om es () = 0 (ep.) 
WL aA Se ay ot Sau 1 0 yal) a 
VAMP aerate nl” < 1 1 od) = 3) 
IX. 1 1 F 1 0 —— a 
aN 1 1 1 Oorr = 3or3+r 
lov. la 1 1 r = 3+r 
MM es, eel! 1 1 r = 3+r 
ie Sy 2 34/0 0 0 1 —— 
6+ep. + 6 ak 5 |= 1 2r or 3r = 18+ 27 o0r37r+ ep. 
The orifice of the green gland is situate on the posterior face of the tu- 
bercle. The annulus ventralis is represented by a transverse ridge behind 
the penultimate thoracie sternum. 
The Astaci occupy three widely separated geographical areas: 1. Western 
North America from the Rocky Mountains to the Pacific Ocean ;* 2. The 
western portion of the Europzo-Asiatic continent, from the Ural Mountains 
and the basin of the Sea of Aral to the Spanish peninsula and Ireland ; 
3. Eastern Asia im the Amoor River system (Transbaikailia, Territory of 
Amoor, and Manchooria), and in Japan. No Astaci are known from any 
part of Siberia between Lake Baikal and the Ural Mountains, or from any 
of the Siberian rivers that flow into the Arctic Ocean.f 
The North American and European Astaci form a natural group (Astacus 
proper). In these the body is robust and ovate, the first pair of abdominal 
* One species, 4. Gambelii, has invaded the territory of the Cambari, following down the Yellowstone 
River to its mouth. 
+ That is, no species known to be indigenous. 4. lepfodactylus has been artificially introduced into the 
Irtish River basin. 
