72 PROCEEDINGS OF THE NATIONAL MUSEUM vol. loi 



The species that Dr. Rathbun (1914, p. 120, pi. 3) described as 

 Planes marinus differs so markedly from the other species of Planes in 

 its wider, subquadrate, and distinctly striate carapace, differently 

 formed chelae, and the absence of a natatory fringe of hairs on the 

 propodi of the first three pairs of walking legs (table 1) that it is here 

 transferred to the genus Pachygrapsus. Except for its much smaller 

 size, this species is very similar to Pachygrapsus crassipes Randall 

 from the Pacific coast of North America and the shores of the Japanese 

 islands and Korea. The only obvious character, apart from size, in 

 which P. crassipes and P. marinus differ is the absence of lateral 

 lobules on the front m the latter species. Unless this species is re- 

 moved from Planes on the basis of the characters mentioned above, 

 the distinction between Planes and Pachygrapsus would cease to exist 

 and Pachygrapsus would have to be synonymized with Planes. 



Examination of one of the two specimens recorded as Planes marinus 

 by Ward (1939, p. 14) indicates that these specimens, like the others 

 mentioned in that paper, are P. cyaneus. 



RELATIVE GROWTH 



The examination and measurement of the large number of specimens 

 of Planes minutus used in this study suggest a problem involving the 

 relative growth of that species which might prove to be of considerable 

 interest if it could be followed up with the analysis of more material 

 from drift logs, turtles, and so forth. 



The carapace length-width relationship remains fairly constant, 

 with a shght tendency toward narrowing, from the smallest immature 

 specimens examined to a carapace length of about 11 mm. At this 

 stage the carapace seems to become somewhat narrower rather 

 abruptly and continues to become narrower at a slightly more rapid 

 rate than during the younger stages (figs. 4, 5). A similar, but even 

 more striking, trend is noticed in the relative shortening of the walking 

 legs (figs. 6, 7). In other words, there is a tendency for the larger 

 specimens of P. minutus to approach the laterally convex, narrow 

 form (fig. 3) and short walking legs of P. cyaneus. These similarities 

 are so striking that I was at first led toward the belief that the larger 

 Atlantic specimens are conspecific with those from the Pacific. It 

 will be seen from figures 4 through 7, however, that the mean width of 

 the carapace in the Atlantic form never becomes quite as narrow 

 relative to the length as in the Pacific form, and the legs are always 

 distinctly longer in the Atlantic species (tables 2, 3). 



Although the material at hand has not been sufficiently well docu- 

 mented to verify the hypothesis, it is not improbable that most of the 

 specimens of P. minutus having a carapace length of more than about 

 1 1 mm. are not found on Sargassum, but on flotsam and turtles. The 



