614 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL.5 



Hancock Expedition of Osachila sona, in every detail a better counterpart 

 of the Atlantic O. antillensis than the currently accepted analogue, O. 

 galapagensis. 



The writer also considers as probably endemic certain insular species 

 now common to Central American mainland shores which evidence shows 

 did not originate there. These are Galapagos species having one or more 

 Pacific congeners, one of which is analogous to the single Atlantic member 

 of the genus. Menippe obtusa and M. frontalis both occur in the Bay of 

 Panama. M. obtusa, found also in the Galapagos, has no Atlantic 

 analogue, whereas M. frontalis, not found in the Galapagos, is analogous 

 to the Atlantic M. mercenaria. The Galapagos origin of M. obtusa 

 would therefore seem to be strongly indicated. Similar examples are 

 Platypodia gemmata and P. rotundata, the latter analogous to the 

 Atlantic P. spectabilis, and perhaps Ocypode gaudichaudii and O. occi- 

 dentalis, the latter analogous to the Atlantic O. albicans. The possible 

 Galapagos origin of the first named in each case has not previously been 

 suggested. 



The foregoing concept leaves the number of Galapagos endemic species 

 unchanged at 23, although it alters slightly the composition of the list. 

 It does, however, strengthen the endogenous nature of this fraction of the 

 Galapagos brachyuran fauna by presenting it as a possible source of enrich- 

 ment of the American mainland fauna. An even stronger argument for 

 the Galapagos origin of certain Central American brachyuran forms will 

 be presented in the discussion of distribution of genera. 



Previously discussed under extra-Galapagan distribution were several 

 species common to the Galapagos and other islands of the Eastern Pacific, 

 particularly Clarion and Socorro, but not to the American mainland. 

 These were grouped with Gulf of California species because, like them, 

 they appear to have been transported to the Galapagos by the California 

 Current and the Equatorial Counteixurrent. That the migration has 

 proceeded predominantly in a southerly and easterly direction is indicated 

 by the prevailing direction of these currents and by the fact that, whereas 

 Gulf of California species are found at Clarion, Socorro, and Galapagos, 

 no Galapagos endemic species has so far been recorded from the Gulf of 

 California. 



A corollary to this assumption would appear to be that species in 

 question must have originated at Clarion or Socorro. This is not im- 

 possible, but is unlikely when one of them, Leptodius cooksoni, is a domi- 

 nant Galapagos shore form said to have analogous Atlantic affinities. 

 These islands, along with San Benedicto and Roca Partida, form the 



