IS 



Subfamily 1.— POTAMONIN^ 

 Genus I. — Potamon, Savigny 



Potamon, Savigny, Mem. Anini. sans Vert., I., 107, 1S16 : Ortmann, Zool. Jalirb., Syst., X., 1S97, p. 299. 



Thelphusa, LatreiUe, Nouv. Diet. Hist Nat., XXXIII., 500, 1819: Milne Edwards, Hist. Nat. Crust., II., 

 p. 10, 1837. 



Potamon, Maht J. Rathbun, Nouv. Arcbiv. du Mus. (4), VI., 1904, p. 247— for full references to literature, 

 but not for the analysis and classification of the genus. 



Carapace oval or subquadrilateral, either flat or convex, traversed anteriorly 

 by a crest, Avhich, on either side of the middle line, consists of two portions, 

 namelj^ — (1) a shorter, coarser, post-frontal or epigastric portion, and (2) a 

 longer, sharper, post-orliital portion : these two portions may be distinct and 

 discontinuous, or may be continuous, or one or both of them may be indistinct 

 to the verge of disappearance (subgenus Geotelphiisa). 



Front always decidedly broader than the orbit, and varying from about 

 two-fifths to about two-sevenths the greatest breadth of the carapace, of 

 varying degrees of declivity, commonly broadly bilobed or sinuous. External 

 orbital angle usually dentiform or spiniform, and usually separated from the 

 lower border of the orbit by a gap. 



Antero-lateral borders of carapace usually well defined, often cristiform and 

 serrulate or crenulate, sometimes (subgenus Acanthotelphusa) cut into large 

 teeth or spines ; their curve is usually broken, somewhere near the level of the 

 post-orbital crest, by a notch and spine — the lateral-epibranchial spine — but 

 these may (subgenus Geotelphiisa) i)e indistinct or obsolete. Postero-lateral 

 borders usually " full " and indefinite, and crossed by oblique wrinkles continued 

 from the sidewalls of the carapace. 



The alidomen of the adult male is regularly triangular, the sides of 

 the segments 3 to 6 being regularly convergent, the segments 5 to 7 not 

 lieing al)ruptly narrowed, the length of the 6th segment much exceeding 

 its distal breadth, and the 7th (terminal) segment being a broad triangle. 

 The abdomen of the adult female is broad, its terminal joint never being 

 elongate. In both sexes all seven segments of the abdomen are distinct and 

 separate. 



The edges of the epistome are well defined : the oral edge is not to any 

 extent everted, so that the efferent branchial openings, though very distinct, 

 are not subtubular. The edges of the meropodites of the external maxillipeds 

 approach close enough to the edge of the epistome to conceal, or almost to 

 conceal, the channel-like laminar terminal joint of the endopodite of the first 

 maxillipeds. 



