XII ALEUTIAN, CALIFORNIAN, PANAMIC SUB-REGION 37 1 



of Cape Cod on the eastern coast, the portions north of these 

 points belonging to the Arctic Sub-region 



(1) The Aleutian Suh-region consists of the islands of Yesso 

 and Saghalien, with the adjacent shores of the Sea of Okhotsk to 

 Cape Lopatka, the Aleutian Is., and the west American coast from 

 about Cape Avinoff (lat. 60° N.) to St. Jean de Fuca Straits. 



A certain number of species, probably of arctic origin, are 

 common with British and also with East American shores, the 

 former being the more numerous. Species as familiar to us as 

 Lacuna divaricata Fabr., Trichotropis horeaUs Brod., Pliolas 

 crispata L., Mya truncata L., M. arenaria L., Mytilus edulis L., 

 and Modiolaria nigra Gray, occur. The more characteristic 

 genera are Chrysodomus^ Volutharpa^ Buccinum, Tectura^ Scumna^ 

 Chiton^ Cryptochiton (^Cr. Stelleri Midd. is by far the largest 

 known of the Chitonidae, 6 inches long), Tellina and Pecten. 



(2) The Calif orniaii Sub-region extends from St. Jean de Fuca 

 Straits (lat. 48° N.) to Cape St. Lucas, the Gulf of California 

 belonging to the Panamic sub-region. The northern polar current, 

 which washes the shores of this sub-region throughout their whole 

 extent, prolongs the southward range of the more northern 

 genera, and keeps back those more markedly tropical, the latter, 

 however, creeping northward in the warmer waters of the Gulf 

 of California. Some authorities subdivide this immense stretch 

 of coast-line, as characterised by sub-temperate, temperate, and 

 sub-tropical genera, into the Oregonian, Californian, and Lower 

 Californian provinces. 



The characteristic genera are — in the north, Argobiiccimim, 

 Zizyphinus^ Chlorostoma, Tectura, Scu7Tia, Chiton (^Katharina^ 

 Mopalia^ Tonicia^^ Cryptochiton^ Placunanomia, ^nd 3Iytilimeria ; 

 in the centre, Purpura^ Monocer as ^ Amphissa^Norrisia^ Platyodon^ 

 Tapes ^ and Macoma ; and, towards the south, Olivella^ Chorus^ 

 Macron^ Pseudoliva^ Trivia, and Haliotis. 



(3) The Panamic Sub-region extends from the head of the 

 Gulf of California to Payta in Peru (lat. 5° S.). It is exceed- 

 ingly rich in species, about 1500 having been described. The 

 MoUusca are entirely distinct from those of the Indo-Pacific 

 Region, which, although extending from Natal to the Sandwich 

 Islands, are unable to pass the enormous extent of sea which 

 separates the nearest Polynesian island from the American coast. 



On the two sides of the isthmus of Panama there occur certain 



