604 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 5 



small but important groups of species remained, the one clearly restricted 

 to the northern and northwesterly islands, the other equally limited to the 

 southern and southeasterly islands. Since these minority groups seemed 

 to offer a clue to routes of arrival and dispersal, it was upon them that 

 attention was particularly concentrated. 



Species found only at the extreme northerly islands were Cymopolia 

 fragilis and C. cortezi, dredged in 100-150 fathoms at Wenman, 

 Microphrys platysonia, dredged in 40-70 fathoms at Tower, and 

 Pilumnus xantusii, cracked from Pocillopora coral at Tower, Albemarle 

 (Albemarle Point and Tagus Cove), and James (Sulivan Bay). These 

 species also occur in the Gulf of California, some 2,000 miles to the north- 

 ward. Another Gulf of California species, Eurytium affine, was collected 

 on the north shore of Indefatigable at Conway Bay. This distribution is 

 shown on Chart I. If these species had been successful in extending their 

 range southward, it was felt that they would in all probability have been 

 taken at one or more of the numerous Velero III stations in the southern 

 half of the island group. 



Species found only at the extreme southeasterly islands were Eupleuro- 

 don rathhunae, washed from sargassum at Hood and Charles, and 

 Microphrys aculeatus, collected on the shores of Hood and Charles and 

 along the south and east shores of Indefatigable to South Seymour and 

 again at Tagus Cove, Albemarle, a distribution shown on Chart II. The 

 latter is a Peruvian species, the former very similar to, if not identical 

 with, Epialtus peruvianus (not Euplcurodon peruvianus) , which should 

 be transferred to Eupleurodon and given another specific name. It was 

 likewise apparent that if these species had been successful in extending 

 their ranges northward they would almost certainly have been collected 

 at one or more of the numerous shore stations in the northern portion of 

 the archipelago. There must be, therefore, some natural obstacle to the 

 free migration of these species from north to south and vice versa, once 

 they have gained foothold at either extremity of the island group. 



It will be seen by an examination of Chart III that the Galapagos 

 Islands are disposed in such a way that the primary channels, those passing 

 from one extreme of the archipelago to the other, are from southeast to 

 northwest. One of these runs on either side of Albemarle, the third 

 between Hood and Chatham, with Barrington, Indefatigable, and James 

 on the west and Chatham, Tower, Bindloe, and Abingdon on the east. 

 An important secondary channel, intersecting at right angles, divides the 

 eastern half of the archipelago north of Indefatigable. It is quite possible 

 that bodies of water of unlike temperatures, salinities, and oxygen contents 



