Acanthogammarus 511 



34-jointed, accessory flagellum 4-joiDted. Antenna 2, flagellum 10-jointed. 

 Gnathopods 1 and 2, 5^** Joint short, 6*^^ tending to oblong, but widening 

 somewhat to the palm, especially in gnathopod 1, which has the palm more 

 oblique than that of gnathopod 2. Peraeopods 3 and 4, 2^ Joint heart-shaped, 

 with long setae on Kind margin. Peraeopod 5, 2*^ Joint well expanded, 

 oblong, but with the wing not quite reaching the top of the S^ Joint, üropod 1 

 not reaching beyond uropod '6. üropod 3, inner ramus very little shorter 

 than outer, both with plumose setae on inner, and simple setae on outer 

 margin. Telson divided. Colour brownish. L. 45 mm. 



Lake Baikal. Depth 100—200 m. 



5. A. armatus (Dyb.) 1874 Gammarus a., B. Dybowsky in: Horae Soc. ent. 

 ßoss., V. 10 suppl. p. 146 t. 12 f. 1 I 1893 Ceradociis? a., A. Della Valle in: F. Fl. Neapel, 

 «.20 p. 927 I 1899 Acanthogammarus a., T. Stebbing in: Tr. Linn. Soc. London, ser. 2 

 V.7 p.430. 



Median carina on peraeon segments 1 — 7 composed of lateral compressed 

 humps, successively larger, on pleon segments 1 — 3 represented by 2 groups 

 of spines, arranged in sets of 3 — 5 on low elevations, on pleon segment» 

 4 — 6 represented only by 4 spines on the bind margin of eacb segment; 

 lateral carinae of peraeon, placed high up on the back, composed of strong 

 ridge or tooth-like prominences, strongly compressed from before backward, 

 directed obliquely to tbe long axis of the animal and bent backward, the 

 Upper rouuded margin beset with 2 — 7 spines. Head smooth, rosti'um short. 

 Side-plates carry only 2 short, spine-like setae. Pleon segment 3, postero- 

 lateral corners rounded. Eyes rather large, oval reniform, slightly prominent, 

 black. Antenna 1 about '^4 as long as body, 4 — 5 times as long as antenna 2, 

 with peduncle stouter and twice as long, 1^* Joint little longer than either 

 2* or 3*^, which are subequal, flagellum 30— 47-joiuted, accessory flagellum 

 3- — 5-jointed. Antenna 2, flagellum 5 — 8-joiuted. Gnathopod 1, H^^^jointpiriform. 

 Gnathopod 2, 6*^1 Joint oblong. Peraeopods 3 — 5, 2'^ Joint heart-shaped, lowerhind 

 comer quadrate. üropod 1 not reaching beyond uropod 3. üropod 3, outer 

 ramus ^/^ — ^/^ longer than inner, both with plumose setae on inner margin. 

 Telson divided, the division (in figure) reaching to the base, the lobes conical, 

 not so long as peduncle of uropod 3. Colour bright horn-brown with darker 

 marbling and a lighter band across peraeon. L. reaching over 24 mm. 



Lake Baikal. Depth 10 m. 



6. A. parasiticus (Dyb.) 1874 Gammarus p., B. Dybowsky in: Horae Soc. 

 ent. ßoss., f. 10 suppl. p. 147 t. 3 f . 3 \ 1893 Ceradocus? p., A. Della Valle in: F.Fl. 

 Neapel, ü. 20 p. 929 | 1899 Acanthogammarus p., T. Stebbing in: Tr. Linn. Soc. London, 

 ser. 2 V. 7 p. 430. 



Median caiina on peraeon segments 1 — 7 forming tooth-like elevations, 

 on pleon segments 1 — 3 keel-like, all beset with a tolerably constant number 

 of spines; the divided tooth of peraeon segment 1 with 3 pairs of 

 spines, segment 2 with 4 or 5, segment 3 with 4 — 6, the rest with 2 or 3; 

 pleon segments 1 — 3 each with 2 spines at the base and several others; 

 pleon segments 4 — 6 without spines; mai'ginal carinae immediately over the 

 side-plates, on peraeon segments 1 — 7 tooth-like elevations, each with 3 — 5 

 spines, on pleon segments 1 — 3 represented by very weakly developed 

 tubercular humps, each with 1 spine. Head with short rostrum, and 

 16 — 18 denticles distributed over the upper surface and sides. Side-plates 

 1 — 4 with only isolated short setae. Eyes roundish, very prominent, black. 



