10 BULLETIN 97, UNITED STATES NATIONAL MUSEUM. 



the various species. One hundred years ago Thomas Say, the pioneer 

 American naturalist, remarked, " It is a curious fact that, although 

 the female of this species is so very often found occupying the oyster, 

 the male is absolutely unknown." 1 The same might almost be said 

 to-day, as up to this time not a single male of that species has been 

 recorded, and only one immature specimen has been seen by the 

 present writer or is known to exist in any collection. 2 



In order to emphasize our lack of knowledge of this important 

 and interesting little family I have, in dealing with the* genus Pin- 

 notheres, made a list of the American species with an indication of 

 the known sex or sexes of each. It is hoped that this will lead to 

 greater interest among workers at the laboratories along our coasts 

 in seeking and preserving examples of Pinnotheridae. They are to 

 be sought for not in the usual haunts of crabs, but in the shells of 

 bivalve mollusks, such as oysters, clams, mussels, and scallops, in 

 the tubes of annelid and sipunculid worms, in ascidians, in the in- 

 testinal tract of certain globular sea-urchins, and on the outer surface 

 of sand dollars and other flat urchins, with all of which animals they 

 may be commensal. In some cases the males, however, may be free- 

 swimming and should be looked for in plankton or tow-net hauls. 



3. The Cymopoliidae are unique in structure, related in most 

 features to the Catometopes, but in the small filiform legs of the 

 last pair resembling the Dorippidae, near which they are grouped 

 by some authors. They come from water of considerable depth and 

 nothing is yet known of their habits or the function of their delicate 

 hind legs, whether used to support a protective covering of sponge 

 or ascidian, as in the Dromiidae, or serving as tendrils to cling 

 to the branches of alcyonarians, hydroids, and algae. 



4. The Grapsidae embrace large numbers of shore and shallow- 

 water crabs, as well as a few which inhabit fresh water or are semi- 

 terrestrial, living at considerable distance from the sea. 



5. The Gecarcinidae are land crabs, often of large size, with 

 smooth, thick carapaces and more or less spinous legs. 



6. The family Ocypodidae is represented in this hemisphere by 

 the sand crabs or ghost crabs and the fiddler crabs (genus Uca, 

 formerly Gelasimus}. In this group the length of the eyestalks and 

 the corresponding narrowness of the frontal projection or rostrum 

 are carried to an extreme, as is also the difference between the cheli- 

 peds of the two sexes. Both chelipeds in the female and one (either 

 right or left) in the male are small, feeble, and similar. The other 

 male cheliped is stout and of enormous length, especially the arm and 

 fingers, the chela (palm and digits) usually much surpassing in 

 length the width of the carapace. 



1 Pinnotheres ostrrum, Journ. Acad. Nat. Sci. Philadelphia, vol. 1, 1817, p. 08. 

 8 In November, 1917, a second male was found In an oyster by W. P. Hay. 



