(Figs 1, 8–24)

Type material. Holotype ♂, BRAZIL, São Paulo: Descalvado, Escaramuça, 25.VI.1944, O. Schubart col. (MNRJ 21420). Paratypes: 3♂, 3♀, same data as holotype (MNRJ 21421); ♂, 2♀, Descalvado, Diamantina, 27.VIII.1944, J. Schubart & O. Schubart col. (MNRJ 11515).

Diagnosis. Pigmentation pale yellow. Eyes with five dark brown ommatidia. Second joint of antennal peduncle with a crest in the outer border. Second joint of antennal flagellum with a faint proximal groove.

Measurements. Male length: 2.75 mm, width: 1.24 mm; female length: 3.03 mm, width: 1.51 mm.

Description. Pigmentation of body faint, pale yellow, head with small brown spots. Eyes dark brown with five ommatidia (Fig. 10). Pereonite Iwith anterior margins reaching the eyes (Fig. 8). Cephalic lateral lobes small, shorter than median lobe which has rounded apex and straight sides (Fig. 9). Pleon slightly narrower than pereon. Pereon, pleon and telson densely provided with fan-shaped scale-setae (Fig. 15). In the posterior margins of pereonites these fan-shaped scale-setae are large and wide, alternating with small and narrow ones, and in the lateral margins they are small. Both median axes of fan-shaped scale-setae evident, as well as the secondary supporting axioles. Nodulilaterales with featureless base. Pereonite VII with only one nodulus lateralis on each side. Position of noduli laterales as illustrated (Fig. 1). Antennula with distal joint with six to seven aesthetascs (Fig. 11). Second joint of antennal peduncle with longitudinal sinuous crest in outer border, more evident in the female (Fig. 12). Left mandible without rods between molar and incisory processes (Fig. 13). Six to seven penicils in molar process of mandibles (Figs 13, 14). Outer group of exite of maxillulae with three teeth and inner group with four undivided teeth (Fig. 16). Maxilla with inner lobe narrower than outer lobe. Sensilla on the inner lobe (Fig. 17). Endite of maxilliped with one small tooth in outer distal border, and with inner distal border smooth (Fig. 18). Pleopods without respiratory areas.

Male. Pereopods I (Fig. 19) and VII (Fig. 20) without apparent sexual dimorphism. Pleopod Iwith heart–shaped exopod (Fig. 21); endopod slender with half distal slightly turned to the outside and simple apex (Fig. 22). Pleopod II with triangular exopod (Fig. 24); endopod with distal half strongly tapered (Fig. 23). Pleopod V with exopod subtriangular.

Remarks. Trichorhina biumbonata sp. nov. is distinguished from the species with four to six ommatidia by the antennal crest, and further as follows: from T. acuta by maxillulae with undivided teeth (two bifid in T. acuta). From T. argentina by the exopod of pleopod Iof male cordiform (ovoid in T. argentina). From T. australiensis by 1) molar process of mandibles with six to seven penicils (one in T. australiensis); 2) seven undivided teeth in maxillulae (in T. australiensis nine, of which two bifid); 3) endite of maxilliped without tooth in inner distal border; with only one in outer distal border (one tooth in inner distal border; one in distal outer in T. australiensis). From T. barbouri by 1) second joint of antennal flagellum of female without groove; 2) tegument smooth. From T. hospes by maxillulae with sevenundividedteeth (in T. hospes nine, two bifid). From T. pallida by 1) molar process of mandibles with five to seven penicils (two in T. pallida); 2) exite of maxillulae with seven undivided teeth (two of inner group bifid in T. pallida). From T. papillosa by 1) absence of papillae on tegument of body; 2) endopods of uropods surpass the tip of telson and of protopods. From T. paraensis by 1) antennula with distal joint with six to seven aesthetascs altogether (five altogether in two groups in T. paraensis); 2) molar process of both mandibles with six to seven penicils (one penicil in T. paraensis). From T. silvestrii by 1) antennula with distal joint with six to seven aesthetascs altogether (four in two groups in T. silvestrii); 2) molar process of both mandibleswith six to seven penicils (three in T. silvestrii). From T. vandeli by 1) maxillulae with seven undivided teeth (in T. vandeli eight, two bifid); 2) endite of maxilliped with one tooth in outer distal border (two teeth in T. vandeli).

.

Etymology. From Latin bi = two + umbo = protuberance, due to the sinuous protuberance forming a crest on the antennal peduncle.