CRUSTACEA MALACOSTRACA. III. 



8 9 



All thoracic segments movable excepting sixth and seventh segments, as these are immovably 

 fused both above and below, the median half of the articulation between them having completely 

 vanished, while a suture or articulation is distinct towards the lateral margin (PI. VIII, figs. 3 a and 

 6a; PL IX, figs. 2 a, 3 a, and 7 a), a structure overlooked by Sars, who has drawn this articulation as 

 if it were complete; the figures cited show, besides, that the articulation between fifth and sixth seg- 

 ments is not straight, but peculiarly bent. The absence or presence of a high tubercle or process, or 

 of two processes, on the posterior part of the ventral surface of the thorax affords excellent specific, 

 but scarcely sexual, characters. — First pair of legs are always a little and sometimes considerably or 

 much thicker than the following pairs; in most species this pair have few spines of very moderate 

 length, but in A", inermis n. sp. and N. crassipes n. sp. they are more or less prehensile organs with 

 long, strong spines on fifth joint. An accessory claw is frequently discernible and is sometimes strong. 

 In the males known to me of three species and in the females of N. inermis n. sp., the three posterior 

 pairs of legs have a number of very or extremely long natatory setae along the upper margin of fifth 

 and sixth joints (PL VIII, fig. 3h; PL IX, figs ia and 4e), while in the males of N. reticidatus n. sp. 

 and N. plebejns n. sp. and in females of all species excepting N. inermis no such setae were found. 



The shape of the posterior part of abdomen differs sometimes in the two sexes (PL VIII, figs. 

 3 i and 3 k). In both sexes a considerable or large part of the ventral surface of the abdomen is not 

 covered by the operculum, which is nearly circular or oblong; in the male the median lamella is some- 

 what broad, and its distal part affords specific characters. The uropods are ventral, moderately small, 

 biramous; the endopod considerably or, rarely, slightly longer than the exopod. 



Remarks. The best generic character is the very curious structure of the distal part of the 

 antenuulae; their terminal vesicle is, as far as I know, without parallel in any other Crustacean. None 

 of the species — excepting N. caspius G. O. S. — is found in low water, and most of them are real 

 deep-sea forms. The males seem generally to be much more rare than the females, but that may be 

 explained by the supposition that most of them, at least to some degree, have natatory habits. I think 

 that future and really thorough deep-sea investigations will lead to the discover}- of a great number 

 of species in all oceans. 



The females of the thirteen species described here, together with N. crassipes 11. sp., may be 

 pivided into two sections, and determined by the aid of the following ke\s. 



Sect. I. Abdominal operculum with an acute process on the lower surface (sometimes this process 

 may be partly broken off, but the irregular shape of the remaining protuberance reveals 

 that it has existed). 



Sect. II. Abdominal operculum without any tubercle or process on the lower surface. 



Section I. 



A. Antero-lateral angles of first thoracic segment terminating in a spine; the angles of second seg- 

 ment with a short, fine seta 1. N. simplex u. sp. 



B. Antero-lateral angles of first segment without any terminal spine, but with or without a fine seta; 

 angles of second segment with a real spine. 



a. Surface of the body without reticulation. 



The Ingolf-Expedttion. III. j. 1 2 



