MARINE GAMMARIDEAN AMPHIPODA 445 



shield-like, not posterodorsally excavate; uropod 3 imiramous; 

 outer lobes of maxilliped vestigial; telson entire; pereopod 3 with 

 article 2 slender. See Amphilochidae, Thaiimatelsonidae, Cressidae, 

 Leiicothoidae, Anamixidae, Phliantidae, Pagetinidae. 



Description. — Rostrum inconspicuous; accessory flagellum absent 

 or 1- or 2-articulate, vestigial; body smooth or carinate; coxa 1 

 small, hidden by following coxae; coxa 4 enlarged, shield-like, not 

 posterodorsally excavate; upper lip incised; mandible with weak, 

 sparsely spinose molar, not triturative, palp absent and when present 

 1-, 2-, or 3-articulate; lower lip usually with inner lobes amalgamated, 

 outer lobes ^ith blunt extremities; maxilla 1 with 2- or 1-articulate 

 palp; maxilla 2 small, stout; maxillipeds slender, ^^ith outer lobes 

 vestigial; gnathopods usually powerful, subchelate, occasionally 

 feeble, gnathopod 1 often simple; uropod 3 uniarticulate, ramus 

 2-articulate; telson of medium length, entire; i^ereopod 3 mth slender 

 article 2. 



RELAnoNSHip. — The Amphilochidae resemble stenothoids but 

 have a biramous uropod 3 and well-developed outer lobes on the 

 maxilliped. 



The Thaumatelsonidae have some urosomal segments coalesced 

 and the telson usually strongly thickened. Perhaps the Thaumatel- 

 sonidae deserve only subfamily status as members of the Stenothoidae. 

 The Cressidae differ from the Stenothoidae by the coalescence of 

 the telson with pleonite 6 and the expanded article 2 of pereopod 3. 



Classification of Genera of Stenothoidae 



The genera of Stenothoidae seem to be segregated from each 

 other by very unsatisfactory means, such as conditions of mandibular 

 palp, maxilla 1, accessory flagellum, and to some extent breadth of 

 article 2 on pereopods 3-5. In other families these characters (see 

 following key) often distinguish species-groups into natural genera 

 but in the Stenothoidae such "pigeon-holing" throws together 

 species with great diversity in gnathopods. Several genera thus 

 represent grades of structure rather than clades. In these small 

 organisms mandibular palps are very difficult to work ^^"ith taxo- 

 nomically as palps are hard to observe and may break off during 

 dissection. There is a strong possibihty that some species may indeed 

 have palps in some populations and lack them in others. It is prudent 

 to mount the head of the animal on a slide and look for the mandib- 

 ular palp before dissection. Often the articulation Une separating 

 palp articles 1 and 2 of the first maxillae is difficult to resolve. 



Shoemaker (1955) in his key to Stenothoidae shows the logic in 

 reestabfishing the genera Prometopa and Microstenothoe which were 



