CRAB GENERA PLANES AND PACHYGRAPSUS — CHACE 67 



Practically none of the references to Planes in the literature are de- 

 terminable to species except on a geographic basis. The synonymies 

 listed below have been compiled more or less arbitrarily, all the rec- 

 ords from the Atlantic and Indian Oceans being credited to P. minutus 

 and those from the Pacific to P. cyaneus. It is very possible that this 

 disposition, particularly as it concerns the Indian Ocean forms, may 

 have to be altered when additional data are obtained. 



This opportunity is taken to thank those persons whose gracious 

 assistance has made this study possible: Dr. Louis W. Hutchins, of the 

 Woods Hole Oceanographic Institution, for furnishing the specimens 

 of Pachygrapsus marinus which prompted the survey; John C. Arm- 

 strong, of the American Museum of Natural History, Dr. Elisabeth 

 Deichmann, of the Museum of Comparative Zoology, and Dr. C. H. 

 Edmondson, of the Bernice P. Bishop Museum, for providing loans 

 of critical specimens; Mr. and Mrs. Boonyong Nikrodhananda, of 

 Bangkok, Siam, for making a cursory survey of drifting objects that 

 might harbor crabs, during an extended transpacific voyage; and finally 

 my colleagues in the U. S. National Museum for their assistance and 

 encouragement during the preparation of this paper. 



MORPHOLOGICAL CHARACTERS 



The species of Planes are extremely variable. This fact has led to 

 the general abandonment of all previous attempts to recognize more 

 than one species in the genus. When Stimpson (1860, p. 231) made 

 the statement that "we are unable to distinguish the specimens 

 [from Baja Cahfornia] from those collected from Gulf weed in the 

 North Atlantic," the conspecific status of the various forms of Planes 

 seemed to be finally established, and few subsequent attempts to de- 

 limit other than the genotype were attempted; even the validity of the 

 very distinct form described as Planes marinus by Dr. Rathbun has 

 been questioned. 



The present study indicates, however, that at least some of the 

 variable characters involved show a discontinuity between the At- 

 lantic and Pacific forms of the genus (table 1) . Probably the most use- 

 ful of these characters is based on the relative lengths of the walking 

 legs. In P. cyaneus from the Pacific the legs are noticeably shorter 

 than in P. minutus from the Atlantic and Indian Oceans. This differ- 

 ence in relative leg length is not so apparent in figure 1 as it would be 

 if specimens of P. minutus and P. cyaneus of similar carapace lengths 

 had been available for figuring. The discrepancy is more obvious in 

 figure 2, d, e, where legs from specimens of similar size are compared. 

 It was found that the simplest way to demonstrate this difference is to 

 compare the combined lengths of the tlu-ee distal segments of the 

 second walking leg with the carapace length (figs. 6, 7). The slight 

 overlap in this character between the Atlantic and Pacific forms may 



