NO. 11 GARTH : DISTRIBUTION STUDIES OF BRACHYURA 613 



rather than between the Bay of Panama and the Caribbean, and an ap- 

 propriate subject for discussion in a paper on distribution of Brachyura 

 of the Gulf of California, rather than of the Galapagos. 



GALAPAGOS ENDEMIC SPECIES 

 In considering the 23 species which have been taken only in waters of 

 the Galapagos Islands, the question of what constitutes a Galapagos en- 

 demic species arises. If a given species is found only in the Galapagos 

 Islands but shows a closer morphological relationship to an Atlantic species 

 than to any Pacific member of its genus, in other words, is geminate, it 

 may be assumed that the two species existed as one in Central American 

 waters prior to the Isthmian closure. In this case the Galapagos species 

 may be expected to occur in Pacific Panamanian waters unless very recent 

 extinction has occurred. If, on the other hand, a species found only in 

 Galapagos waters shows no greater affinity to a Caribbean species than to 

 the nearest Pacific member of its genus, i.e., is not geminate, it may be 

 assumed that it was not an inhabitant of Central American coastal waters 

 in Pliocene times. It is evident that such a species must have occupied the 

 Galapagos Islands for a period of time sufficient to have become specific- 

 ally distinct from either the American or Asiatic stock^ from which it was 

 originally derived. It is to the latter group that the writer would apply 

 the word endemic, in a more restricted sense than usual, indicating that 

 the species may be demonstrated to have originated in the archipelago by 

 evidence other than its having been taken only there. 



It is obvious that no true Galapagos endemic species should have an 

 Atlantic analogue. However, Mithrax bellii, Portunus angustus, Actaea 

 angusta, and Osachila galapagensis, taken only in the Galapagos to date, 

 are said by Rathbun (1925, 1930, 1937) to possess such analogues. Fol- 

 lowing the reasoning of the last paragraph, these species, if true analogues, 

 should be expected to occur in the Bay of Panama. However, the dis- 

 covery in Pacific Panamanian waters of species which could be demon- 

 strated to be even more closely related to the Atlantic representatives than 

 the Galapagos species would resolve the question equally well, but in 

 favor of the endemic nature of the latter. Such a solution undoubtedly 

 occurred with the discovery at Bahia Honda, Panama, by the 1939 Allan 



1 Lest it be thought that the 23 Galapagos endemic species of Brachyura have 

 been derived in whole or large part from Indo-Pacific sources, it should be stated 

 that 9 are members of the Majidae, a family strictly of western hemispheric 

 origin, while 7 of the 9 endemic Xanthidae can be traced to a more nearly related 

 American than Old World species. 



