MARINE GAMMARIDEAN AMPHIPODA 333 



Centromedon Sars 



Centromedon Sars, 1895. — Stebbing, 1906. 



Type-species: Anonyx pumilus lAliehovg, 1865 (monotypy). 



Upper lip and epistome small, but separated by a notch, upper lip 

 slightly projecting; mandibular molar conicolaminate, unridged, palp 

 attached level with molar; maxillipedal palp article 4 very small; 

 gnathopod 1 poorly subchelate, articles 5 and 6 equal; coxa 1 slightly 

 tapering distally; telson of medium length, deeply cleft; head small, 

 lateral lobes acute (Hirondellea) , mouthparts hidden by coxae. Species: 

 1, subarctic and boreal N. Atlantic, littoral. 



Cheirimedon Stebbing 



Cheirimedon Stebbing, 1888; 1906. 



Type-species: C. crenatipalmatus Stebbing, 1888 (monotypy). 



Upper lip and epistome not differentially produced; mandibular 

 molar very small, sharp or blunt, ridged (type) or poorly ridged and 

 sublaminate, palp attached level with molar; gnathopod 1 subchelate, 

 article 6 broadly expanded (type) or poorly expanded, occasionally 

 slightly chelate in appearance; coxae normal (type) or coxa 2 con- 

 cealing part of slightly shortened (and occasionally strongly narrowed) 

 coxa 1; telson of medium length, deeply cleft (type) or cleft as 

 little as one third. Some species resembling Anonyx, Boeckosimus 

 { = "Onisi'mus" auct.), Schisturella {= Pseud onesimus), and Tryphosella 

 (incl. " Try phosa" &uct.). Species: 5, amphiboreal, littoral to bathyal. 



The type-species, Cheirimedon crenatipalmatus Stebbing (1888) is 

 characterized by: lateral outline of mouthpart group quadrate; 

 epistome and upper lip not differentially produced; head normal; 

 mandibular palp attached level with molar, molar process columnar, 

 triturative; other mouthparts normal; coxa 1 quadrate, not narrowed 

 distally or covered by coxa 2; gnathopod 1 subchelate, with article 

 6 longer than 5 and slightly expanded distally; telson cleft more than 

 three fourths of its length; uropod 3 normal, outer ramus 2-articulate. 



Cheirimedon crenatipalmatus thus differs from Anonyx in (1) the 

 nonsetulose mandibular molar which in Anonyx is bulging or sub- 

 laminate but lacking ridges and teeth and (2) in the expanded article 

 6 of gnathopod 1. It differs from Tryphosella {—"Tryphosa" auct.) 

 in the nontapering coxa 1, and the distally expanding article 6 of 

 gnathopod 1. The type-species is joined by C jemoratus (Pfeffer) { — C. 

 dentimanus Chevreux, 1906b) which is distinguished generically only 

 by the telson being cleft halfway and by the poor expansion of gnath- 

 opod 1. 



The other species that have been assigned to this genus depart 

 radically from the above brief diagnosis. Cheirimedon latimanus 



