56 CRUSTACEA MALACOSTRACA. III. 



ments, but all thoracic segments excepting the first which is fused with the head -- are free and 

 movable. 



Some other good characters between the three genera can be pointed out, but they are found 

 in the brief descriptions of the genera later on. But it may be useful to give an analytical key to 

 the three genera; the fourth genus, Rhabdomesus Rich., is still somewhat doubtful, as its two "Chal- 

 lenger" species were established on specimens without the anterior part of the body, and judging from 

 the figures of their abdomen the genus must be cancelled and its species referred to Ischnomesus. 



A. The three posterior thoracic segments movable; the abdomen consists of two movable segments. 

 Uropods two-jointed. Third joint of the antennal peduncles moderately short, not twice as long as 

 the fourth Ischnomesus Rich. 



B. At least the two posterior thoracic segments and the abdomen immovably fused, constituting a 

 single piece. Uropods one-jointed. Third joint of the antennal peduncles elongate, much more than 

 twice as long as the fourth. 



a. The three posterior thoracic segments completely fused. Body slender to extremely slender. 

 Antennulae with third peduncular joint and the 3-jointed flagellum well developed. 



Haplomesus Rich. 



b. Only the two posterior thoracic segments and the abdomen fused, as a real articulation is found 

 between fifth and sixth segment Body moderately slender or proportionately robust. Antennulae 

 with the part beyond second joint reduced, consisting of two or three small or quite rudi- 

 mentary joints Heteromesus Rich. 



IschnomeSUS Richardson. 



Body slender or very slender. Antennulae well developed, 6-jointed. Antennae with third joint 

 moderately short, not twice as long as the fourth. Maxillipeds (PI. IV, fig. 5 c) with second joint some- 

 what large, its lobe at least as long as broad, and less than half as broad as the joint; the palp at 

 least as long as second joint itself - - the lobe not included -- and its second and third joints are 

 considerably expanded, conspicuously broader than the lobe mentioned. - The posterior thoracic seg- 

 ments movable. First pair of thoracic legs with the fifth joint much deeper than the fourth, having 

 its proximal half much expanded, and being broadest before the middle. Abdomen consists of two 

 movable segments; the first is very short. - - Uropods two-jointed, sometimes with a quite small exopod 

 from the end of first joint. 



The "Ingolf" has secured two species, both new. 



34. Ischnomesus profundus n. sp. 



(PI. IV, figs. 5 a- 5 f). 



Male. Body about six times as long as the breadth of first thoracic segment. -- Antennulae 

 somewhat longer than the breadth of first segment; third joint half as long as second and as long as 

 the flagellum (fig. 5 b). Third joint of the antennal peduncle about half as long again as the fourth 

 (the following joints lost). 



