PANAMIC-PACIFIC PELECYPODA 25 



The northern hmits of the Panamic-Pacific faunal region can be drawn 

 closely since it lies at the head of the Gulf of California at approximately 

 lat 30° 30' N. and about long 114° W. From this point, the province extends 

 in a southeasterly direction, its waters bathing the coast of Mexico, Guate- 

 mala, Nicaragua, Costa Rica, Panama, western Colombia, Ecuador, and 

 northwestern Peru, or for a distance of about 4500 statute miles. It may 

 also be somewhat expanded so as to include the waters around such off- 

 shore islands as Tres Marias, Cocos, and the island archipelago of the 

 Galapagos. At the south, its limits cannot be fixed so closely and opinion 

 has varied as to its location. For geographic reasons, the south boundary 

 seems best placed at Cabo Blanco at the lower end of the Gulf of Guaya- 

 quil and the most westerly point of the South American continent. For the 

 same reason, the north border of the Peruvian Province is placed at Punta 

 Aguja (Dall location) at the southwest end of the Bay of Sechura, thus leav- 

 ing between the two faunal regions a transitional zone about 120 nautical 

 miles wide, here called the "Paita Buffer Zone" after the city and Bay of 

 Paita situated near its middle point. 



The Panamic-Pacific faunal region is essentially a tropical one as would 

 be expected from its geographic location, but at many places strong up- 

 wellings occur which materially lower the temperature of the surface waters. 

 Such belts of relatively cooler waters are met with frequently in the Gulf 

 of Panama and off certain sections of Costa Rica. They periodically appear 

 along the coast of Ecuador and when severe or of long duration may bring 

 to those sections of the coastal lands a climatic upset similar to that of the 

 Humboldt Current much further south in Peru. In Ecuador, as in Peru, the 

 winds blowing steadily from the sea onto a more highly heated land, become 

 dry and desiccating, and produce belts of aridity along the coast, in some 

 cases nearly as barren and desert as in Peru (Santa Elena, Isla la Plata, 

 Manta). Where the coast trend offers shelter from the sea winds, the climate 

 is more humid and wet, the hills are clothed with vegetation, and the shore 

 itself lined with mangrove (the inner section of the Gulf of Guayaquil. 

 Manglaralto, Salango). 



Although the waters of the Panamic-Pacific Province are somewhat 

 cooler than would normally be expected in these latitudes, its fauna is 

 essentially a tropical one but much less so than that of the Indo-Pacific, as 

 shown by the general absence of reef corals and in the mollusks by the fewer 

 species of Conus, Cypraea, and others. It is probably due to this factor that 

 so few Indo-Pacific species have been able to establish themselves along the 

 mainland coast although many such species have been able to do so on 

 some of the off shore islands as reported by Hertlein. 



In spite of its great length, the fauna of the Panamic-Pacific Province 

 is remarkably uniform throughout with many of its most common species 

 ranging from the Gulf of California southward to Peru. This fact is partic- 

 ularly well illustrated by the Pelecypoda as shown by the tabulation on 

 page 26. These percentages are based upon existing data and can be expected 

 to change as the range of some species becomes better known. 



