34 AXEL A. OLSSON 



meters only. It brings to the surface an abundant supply of mineral salts 

 and nitrogenous compounds favorable to the growth of an extraordinarily 

 rich planktonic fauna and flora which in turn gives support to vast schools 

 of anchovies and other small fish. These are the main source of food 

 for the vast swarms of guano birds seen along the Peruvian coast south 

 of Sechura. Through most of the year conditions in the path of the Peru- 

 vian Current are fairly stable but during the summer months of the 

 Southern Hemisphere upsets in circulation may occur, the upwellings may 

 diminish or cease entirely, and the invasion of a southward flowing, inshore 

 current, the "El Nino" and the accompanying phenomenon known as the 

 "Aguaje" or Callao Painter sets in as already mentioned. In most years, 

 the effects of the El Nino are relatively unimportant and of short duration 

 but during some years, spaced roughly at 7 and 35 year intervals, the cli- 

 matic upset of the El Nino may be great. In the Aguaje of 1925, the tem- 

 perature of the surf at Negritos in northwestern Peru rose sharply from an 

 average of about 65° to about 80° and remained so for several weeks. At 

 the same time, surface water temperatures in the harbor of Callao were 

 reported as high as 80 degrees. Such a sharp rise in the temperature of 

 the sea water, if prolonged, results in a catastrophic destruction of most 

 forms of marine life along large parts of the Peruvian coast as well as an 

 upset in the climatic pattern on land. At such times, the decomposition of 

 the vast quantity of dead marine life, much of it cast upon the beach, 

 generates volumes of hydrogen sulphide that blackens the walls of houses 

 and hulls of ships which are painted with white lead (the Callao Painter). 

 Evidence of the southward flowing El Nino current in 1925 was demon- 

 strated by the large numbers of sea-snakes stranded on the beach at 

 Negritos and of mangrove seedlings rooted in tidal lagoons and flats as 

 far south as Sechura. 



An estimate of the size of the molluscan element in the Peruvian fauna 

 is still difficult to make as many records in the literature cannot be relied 

 upon with full confidence. Fully half of the species so far known from Peru 

 and middle Chile are littoral forms such as the limpets, chitons, and other 

 rock perching types and show that the offshore species are known only 

 in an imperfect manner. The first compilation of the mollusks of Peru and 

 Chile was made by Alcide d'Orbigny and published in the "Voyage dans 

 I'Amerique Meridionale" (1835-1843). In this list D'Orbigny recorded 251 

 species as living along the coast from southern Chile to the Peruvian-Ecua- 

 dorian border. Of this number, 89 species were listed from Paita or from 

 Tumbez, and of which only one species was also known to occur at Callao. 

 These northern species are members of the Panamic-Pacific and the Paita 

 Buffer Zone, leaving only 163 species for the restricted Peruvian fauna as 

 then known. In 1909, our knowledge of the Peruvian fauna was greatly 

 advanced by Dall through his work on the Coker Collection as noted in 

 an earlier section of this paper. 



The compilation following was prepared largely for this occasion and 

 is based on our own collections and observations and on such records in 



