460 Dikerogamraarus, Gammarus 



5. D. faseiatus (Say) 1818 Gammarus f., Say in: J. Ac. Philad., i\lii p. 374 j 

 1830 G.f., H. Milne Edwards in: Ann. Sei. nat., t;.20 p. 367 | ?1862 G.f., Bäte, Cat. 

 Amphip. Brit. Mus., p. 210 t. 37 f. 6 | 1874 G. f., S. I. Smith in : Rep. U. S. Fish Comm., 

 V.2 p. 653 I 1899 Dikerogammarus f., T. Stebbing in: Tr. Linn. Soc. London, ser. 2 

 V.7 p.428. 



Pleon Segments 4 aud 5 slightly angulated dorsally at bind margiu, 

 each armed with 3 fascicles of spines, the median raised ou a distinct protu- 

 berance, segment 6 also with 3 fascicles. Antenna 1, 2*^ Joint suiTOunded 

 with long setae, which reach the 5*^ Joint of flagellum, accessoiy flagellum 

 5- or 6-joiuted. Antenna 2 setose. Gnathopod 1 in d, 6*^ Joint much 

 naiTOwed distally, palm very oblique, defined from the spineless bind margin 

 by small spines, and having a stout spine at the middle, in Q only slightly 

 naiTOwed distally, palm less oblique. Gnathopod 2 in c5, ^^^ Joint rather 

 longer than in gnathopod 1, 6*^ with margins nearly parallel, but widening 

 a little to the palm, which is a little oblique, slightly concave, with a lamellar 

 edge, carrying a stout spine near the middle, and defined by groups of small 

 spines; in 9 5*^ Joint proportionally longer, as wide as the uarrow, parallel- 

 sided 6*^, which has a short. nearly transverse j)alm. Uropod 3, inner ramus 

 usually with 1 or 2 spines on the inner margiu. Telson, each lobe with a 

 spine and 1 or 2 hairs on outer margin as well as a few spines and hairs 

 at the apex. L. 10 — 15 mm. 



Northern United States of America. Streams and ponds. 



44. Gen. Gammarus F. 



1758 Cancer (part.), Linne, Syst. Nat., ed. 10 p. 625 1 1775 Gammarus (part.), 

 J. C. Fabricius, Syst. Ent., p.418 | 1813/14 G., Leach in: Edinb. Enc, v.7 p. 402, 482 | 

 1862 G., Bäte, Cat. Amphip. Brit. 3Iiis., p. 203 ] 1874 G. (part.), B. Dybowsky in: Horae 

 Soc. ent. Boss., r. 10 suppl. p. 19 i 1876 (?., A. Boeck, Skand. Arkt. Amphip., v.2 p. 364 | 

 1886 G., Gerstaecker in: Bronn's Kl. Ordn., i;. 5ii p.511 | 1888 G., T. Stebbing in: 

 Bep. Voy. Challenger, v.29 p. 1005, 1673 | 1893 G., A. Della Valle in: F. Fl. Neapel, 

 «.20 p.756 I 1894 G., G. O. Sars, Crust. Norway, v.l p.496 | 1793 [Subgen.] Gammarellus 

 (part.), J. F. W. Herbst, Naturg. Krabben Krebse, v. 2 p. 106. 



Body without carinae, teeth or tubercles; dorsal spinules generally in 

 median and lateral groups on pleon segments 4 — 6, but sometimes wanting on 4*^ 

 or on both 4*^ and 5**'. Read without conspicuous rostrum. Side-plates 1 — 4 

 usually rather deep. Eyes present. Antenna 1 almost always longer than antenna 2, 

 though with shorter peduncle, accessory flagellum almost always with more 

 than one Joint. Antenna 2, flagellum not- or little louger than peduncle. 

 Upper lip as broad as deep, faintly emarginate at apex. Lower lip without 

 inner lobes. Mandible normal. Maxiila 1 (Fig. 87 p. 476), inner plate with several 

 setae, outer with (11?) more or less serrate spines, 2"^ Joint of palp with 

 spine-teeth on the apex of one maxilla and slender spines on the other. 

 Maxillipeds normal, Gnathopods 1 and 2 subchelate, strouger in ö than 

 in Q , gnathopod 2 usually the larger, in the d often much the larger. 

 Peraeopods 3 — 5, 2'^ Joint variable, much or little expanded. Branchial 

 vesicles simple, with narrow attachment. Uropod 3, outer ramus usually 

 long and having a small 2'^ Joint. Telson cleft, usually to the base. 



30 accepted species, 1 doubtful. 



Synopsis of accepted species: 



Antenna 1, accessory flagellnm of only one Joint 1. Gr. guernei p. 462 



Antenna 1, accessory flagellum of more joints 

 than one — 2. 



,1 



