184 BULLETIN 97, UNITED STATES NATIONAL MUSEUM. 



Abdomen in both sexes consisting of 7 separate segments. In the 

 female the genital openings are on the second segment of the sternum 

 close to the suture between it and the first. 



Habitat. Taken on a variety of bottom mud, sand, coral, rocks, 

 broken shells, etc. The special use of the small, delicate, hind feet is 

 not known. The carapaces are usually free from foreign substances ; 

 occasionally one bears an encrusting tube of a Serpulid. A very few 

 instances of parasitism have been noted in the attachment of a 

 Ehizocephalid to the abdomen. 



Distribution. Atlantic coast of America from off the southern 

 coast of New England to Cape Frio, Brazil; eastern Atlantic and 

 Mediterranean; Indo-Pacific from the western Indian Ocean to the 

 Hawaiian Islands ; Pacific coast of America from Gulf of California, 

 Mexico, to Ecuador. 4 to 298 fathoms. 



Characteristics of the American species. The genus is divided into 

 several groups ranging from the typical one, with rather flat, squarish 

 carapace, with large and usually thin antero-lateral teeth which seem 

 to form an integral part of it, and with legs of moderate length (not 

 over twice as long as width of carapace), to the group with very 

 convex, swollen carapace, small, thick, tuberculiform or spiniform, 

 distant, antero-lateral teeth which project outside the periphery of 

 the carapace, and with long slender legs (ranging from twice to 

 three and one-half times width of carapace). To the first or typical 

 group (1) belong cristatipes, altemata, sonata, lucasii, faxoni, and 

 affinis. To the second or convex, long-legged group (2) belong 

 cursor, gracilis, floridana, graciUpes, and acutifrons. 



Between these extremes are various modifications as follows: 



3. The group of rathbuni, 'bahamensis, and isthmia^ which have 

 small antero-lateral teeth, distant from each other, or from the orbit, 

 or both ; combined with a typical carapace and short legs ; the three 

 species agree in having the two frontal lobes very feebly subdivided. 



4. Another small group allied to the typical one but with broad, 

 projecting plates of abdomen and sternum which are conspicuous in 

 dorsal view ; this group includes sica, angusta, and depressa. 



5. A group with convex carapace, prominent tubercles, approach- 

 ing in aspect the most atypical group, but with short legs : dentata, 

 obesa, tuberculata. 



6. One long-legged form (fragilis} has a flatter carapace and 

 larger, thinner, antero-lateral teeth than in the cursor group. 



ANALOGOUS SPECIES ON OPPOSITE SIDES OF THE CONTINENT. 



Atlantic. Pacific, 



alternate. zonata. 



ftutxml. lucaaii. 



obesa. tuberoulata. 



