284 BULLETIN 97, UNITED STATES NATIONAL MUSEUM. 



■will again saturate itself with oxygen, and may then serve afresh 

 for the purpose of respiration. In order to complete this arrange- 

 ment the ridges on the right and left sides of the outer maxillipeds 

 form together a triangle with the apex turned forward — a break- 

 water by which the water flowing from the branchial cavity is kept 

 away from the mouth and reconducted to the branchial cavity. In 

 very moist air the store of water contained in the branchial cavity 

 may hold out for hours, and it is only when this is used up that the 

 animal elevates its carapace in order to allow the air to have access 

 to its branchiae from behind. 



Genus SESARMA Say. 

 MARSH CRABS (F. MuUer). 



Sesarma Say, Journ. Philadelphia Acad. Nat. Sci., vol. 1, 1817, p. 76 ; type, 



S. reticulatum (Say). 

 Pacliysoma de Haan, Fauna Japon., Crust., 1833, p. 5; 1835, p. 33; type, 



P. bidens de Haan. Not Pachysoma MacLeay, 1821, a genus of Coleop- 



tera. 

 Chiromantes Gistel, Natur. Thierreichs, 1848, p. X; type, C. bidens (de 



Haan). 

 Holometopus Milne Edwards, Ann. Sci. Nat., ser. 3, Zool., vol. 20, 1853, 



p. 187 [153] ; type, H. haematocheir de Haan. 

 Holograpsus Milne Edwards, Arch. Mus. Hist. Nat. Paris, vol. 7, 1854, 



p. 158. A slip of the pen for Holometoptis. 

 Sesarma de Man (subgenus), Zool. Jahrb., Syst., vol. 9, 1895, p. 143; type, 



H. haematocheir de Haan. 

 Episesarma de Man (subgenus), Zool. Jahrb., Syst., vol. 9, 1895, p. 165; type, 



S. tctragonum (Fabricius). 

 Parasesarma de Man (subgenus), Zool. Jahrb., Syst., vol. 9, 1895, p. 181; 



type, <S. qnadrata (Fabricius) =C'«nccr quadratus Fabricius, 1798, not 



1787=S. pUcaUis Latreille, 1802-1803. 

 Perisesarma de Man (subgenus), Zool. Jahrb.. Syst., vol. 9, 1895, p. 208; 



type, S. bidens (de Haan). 



Carapace squarish; sides usually straight and sometimes parallel, 

 but sometimes convex; surface flattened; gastric region well delim- 

 ited, divided into five subregions, of Avhich the four antero-lateral 

 subregions form four prominent postfrontal tubercles. Postero- 

 lateral regions usually crossed by oblique parallel ridges. 



Side walls finely reticulate; due to a multitude of small uniform 

 granules arranged in pairs in close-set parallel rows; between the 

 pairs of granules is a little row of bristles, one of which in each row 

 is long and points diagonally forward. 



Front equals half or more than half of the anterior border and is 

 obliquely or vertically deflexed. The deep oval orbits occupy the 

 rest of the anterior border of the carapace; below their outer angle 

 is a deepish gap leading into a system of grooves which open into a 

 notch at the antero-lateral angle of the buccal cavern. 



