610 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 5 



cooksoni, and Eriphia granulosa are to all intents and purposes Galapagos 

 endemic species. Of the remaining five, Pinnaxodes chilensis is com- 

 mensal with a common Peruvian sea urchin, Strongylocentrotus gibbosus. 

 The free-living Eupleurodon rathbunae and Microphrys aculeatus, the 

 distribution of vi^hich has been discussed, as vi^ell as Taliepus marginatus 

 and Pisoides edwardsii, unrecorded from the islands since Cuming's time, 

 are found on brown algae, a fact which lends credence to the drift method 

 of arrival via the Peruvian Current. Contrary to its magnitude, this 

 current appears to have exerted but a minor influence on the composition 

 of the Galapagos brachyuran fauna. This is no doubt due to its seasonal 

 and cyclical displacement from Galapagos waters by Nino Current and 

 Equatorial Countercurrent phenomena, as previously explained, which 

 acts to prevent the poikilothermic portion of the Subantarctic fauna from 

 becoming permanently established in the archipelago as have the holo- 

 thermic penguin and fur seal. 



ATLANTIC ANALOGUES 

 (See also Table of Distribution) 



As indicators of former geologic relationships in the Central American 

 region, second only in importance to those species which occur on both 

 sides of the Isthmus of Panama are those which occur in the Pacific only 

 but which have counterparts in the Atlantic from which they are scarcely 

 distinguishable. Such species are known as "twin", geminate, or repre- 

 sentative species and are more closely related than are ordinary species. 

 Their occurrence has been considered among echinoderms by Verrill 

 ( 1867) , among brachyurans by Rathbun ( 1899 et seq.) and among fishes 

 by Jordan ( 1908), to mention only a few. They are considered as valid 

 evidence that the Atlantic and Pacific Oceans were confluent at a com- 

 paratively recent time, probably lower Pliocene (Vaughan, 1919). Charts 

 VIII, IX, and X, after Schuchert (1935), show the location of three 

 such oceanic connections since middle Oligocene. 



A total of 38 Galapagos brachyuran species are considered by Rathbun 

 ( 1918, 1925, 1930, 1937) to possess such analogues. This fraction, which 

 represents over 30 percent of the number recorded for the islands, is the 

 largest segment of the brachyuran population to which a common origin 

 can be assigned. It indicates an older and more fundamental relationship 

 between faunas than that which is based upon the occurrence of the same 

 species in each of them. It also suggests the former existence of a strong 

 current flowing in a westerly direction through the Panama Portal which 

 may have served for the transportation of larval crustaceans in the same 

 manner that the California, Nino, Peruvian Currents, and Equatorial 

 Countercurrent appear to serve today. 



