INTRODUCTION. 



ON THE MARINE ZOOLOGICAL PROVINCE OF PANAMA. 



Panama is situated near the middle of a well defined 

 marine zoological province. Perhaps none of the species of 

 testaceous MoUusca, (to which part of the Fauna our remarks 

 are limited,) which inhabit the neighboring seas, exist south 

 of 22° S. lat., or north of 28° N. lat., or west of the Gallapago 

 Islands. All of the few examples of species, which are sup- 

 posed to have a wider range, are more or less doubtful. Some 

 species which inhabit the northern part of the province, and 

 others which inhabit the southern part, may overlap the 

 boundaries between this and the adjacent provinces. But 

 these species present only the usual difficulty in attempting 

 to define the limits of a zoological province. 



The most definite and satisfactory method of defining the 

 limits of this province, is to place the boundaries at the ex- 

 treme limits of the range of about 99 per cent, of the 

 species which inhabit its middle regions. Thus it will be 

 seen that several of the species which inhabit Panama also in- 

 habit Gruaymas, in the G-ulf of California, nearly in 28° N. 

 lat. ; but none of them inhabit San Diego, which is near 33° 

 N. lat. In the same manner the southern limit is found near 

 the boundary between Peru and Chili, between 22° and 24° 

 S. lat. 



The reason why the range of the species south of the 

 equator is several degrees less than on the north side, is ob- 

 vious in the Antarctic current, which sets along the west 

 coast of South America. In like manner, on the eastern 

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