174 THE VOYAGE OF H.M.S. CHALLENGER. 



ance of tlie male at once distinguishes this genus from all other known Mysidans, the 

 sexual characters being quite anomalous and differing from what is usually met with in 

 this fomily. Moreover, the structure of the antennal scale and of the telson, as also that 

 of the outer plates of the uropoda, would seem to afford well-marked characters, distin- 

 guishing this genus from the genus Boreomysls. As regards the imperfect development 

 of the eyes, which has suggested the genei'ic name, that character is also found in one of 

 the species belonging to the above mentioned genus, viz., in Boreomysis scyphops, 

 G. 0. Sars, to be described in the sequel. 



42. Petaloplithahnus armiger, Willemoes-Suhm (PI. XXXII. figs. 1-9). 



Petalophthalmus armiger, Suhra, Trans. Linn. Soc. Loud., ser. 2, vol. i. p. 40, pi. viii. 



Specific Characters. — Male : Body rather slender, well-nigh cylindrical in form. 

 Carapace remarkably small and not nearly covering whole of trunk, two segments of 

 which are entirely exposed ; cervical sulcus well marked ; frontal margin forming, in the 

 middle, only a very small projection ; antero-lateral corners narrowly rounded. Eyes 

 forming two very thin oval lamellfe, suj)ported on short pedicles. Antennular peduncle 

 exceedingly slender, with basal joint longer than the two others taken together, fiagella 

 shorter than peduncle, subequal. Antennal scale narrow, lanceolate, projecting but 

 slightly beyond basal joint of antennulse ; fiagellum imperfectly developed, shorter than 

 scale, biarticulate. Mandibular palp projecting beyond antennular peduncle, last joint 

 reflexed and armed with seven strong spines. Maxillipeds short and thick, without 

 exopodite, meral joint exj)anded interiorly to a linguiform lobe, terminal joint unguiform. 

 Gnathopoda of a structure similar to maxillipeds, but much larger. The three anterior 

 pairs of legs with last joint densely hirsute ; remaining pairs very slender, with terminal 

 part not subdivided, and having a distinct but feeble claw. Telson oblongo-quadrangular, 

 apex broadly truncate, jutting out in the middle as a short dentiform projection, and 

 having on either side five strong spines ; lateral edges finely denticulate in the outer half. 

 Terminal joint of outer plates of uropoda very sharply defined, linguiform. Length, 40 mm. 



Remarhs. — Of this interesting form I have only had opportunity of examining a 

 single male specimen, apparently that described by the late Dr. v. Willemoes-Suhm. 

 That author has also described the female ; but the specimen from which his descrip- 

 tion was drawn up has unfortunately been lost. To judge from the figures given, it 

 would seem to have presented a far less deviating appearance, resembling rather the 

 species of the genus Boreomysis. 



Description of the Male.- — The specimen examined by me, which apparently is full- 

 grown, measures 40 mm. in length. 



The form of the body (see PI. XXXII. fig. 1) is very slender and well-nigh cylindrical, 

 without exhibiting any sharp demarcation between the anterior and posterior divisions. 



