70 PROCEEDINGS OF THE NATIONAL MUSEUM vol. loi 



The shape of the carapace is much more variable in P. minutus than 

 in P. cyaneus, but much of the variation in the former species is corre- 

 lated with size (fig. 3). In the smallest specimens of P. minutus ex- 

 amined (carapace length 3.8 to 4.5 mm.) the carapace is nearly square. 

 At a carapace length from about 4.5 to 5.5 mm., the carapace becomes 

 more or less convex laterally, as in P. cyaneus; this stage is smaller than 

 any specimens of the Pacific species examined, however, and so there 

 is httle difficulty in separating the young stages of the two species. 

 Following this stage the carapace becomes narrowed posteriorly and 

 assumes a more or less trapezoidal shape; this shape persists in most 

 specimens up to those with carapace lengths of 11 or 12 mm. and is the 

 form commonly found living on Sargassum. As the crab becomes 

 larger than this, the carapace becomes more convex laterally and less 

 narrowed posteriorly and thus closely resembles P. cyaneus once more. 

 No such polymorphism has been noted in the latter species; in that 

 form the carapace is laterally convex at all sizes. The front is often 

 more deeply excavate in P. cyaneus than in P. minutus, but this char- 

 acter is extremely variable. 



The shape of the male abdomen, one of the characters upon which 

 Dana based P. cyaneus, is usually distinctive in the two species (fig. 

 2,g,h). The lateral margins are less sharply convergent from the 

 third to the sixth segment in P. cyaneus than in P. minutus, thereby 

 giving the abdomen of the former a narrower appearance. The 

 terminal segment is in the form of a nearly equilateral triangle in P. 

 minutus, whereas its lateral margins are noticeably convex in P. 

 cyaneus. 



No consistent differences in the form of the male abdominal append- 

 ages could be found. The apparent dift'erences indicated in figure 

 2,j-r, are probably no greater than may be found by examining first 

 pleopods from several individuals of the same species. Even the form 

 of that appendage in Pachygrapsus marinus is of questionable value as 

 a taxonomic character. 



From the material available the two forms of Planes appear to be 

 sufliciently well marked to be recognized as distinct species. Addi- 

 tional material, particularly from the South Atlantic, Indian, and 

 western Pacific regions should be examined, however; if intermediate 

 forms are found in those areas it may be advisable to reduce P. cyaneus 

 to subspecific status. As mentioned in the remarks on relative growth 

 (p. 73), there is some indication that two forms may be represented 

 in the Atlantic: a small trapezoidal one living on floating weed and a 

 larger one, approaching P. cyaneus in form, found on turtles, floating 

 logs, and other flotsam. Additional specimens of various sizes from 

 the latter habitat should be examined to determme whether this form 

 is distinct or not. 



