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



This genus differs considerably from Penseus in the consistency of the general surface, 

 which is soft and yielding, and more like flexible membrane than the shell of a 

 crustaceous animal. The only rigid parts are the appendages and the caudal extremity. 



The species of this genus may generally be recognised by the character of the 

 rostral crest, which is shorter than in either Penseus or Aristeus. The form of the 

 ophthalmopod also differs ; it is flattened and two-jointed in Penseus, cylindrical and 

 single-jointed in Aristeus, and single-jointed and flattened in Benthesicymus, where it 

 also carries a tubercle similar to that in Aristeus and other genera. Owing to the 

 semitransparent character of the superficial tissue in Benthesicymus I have been able to 

 determine that this tubercle is a rudimentary, or rather a complementary, eye, a branch of 

 the optic nerve leading directly to it from a ganglion situated within the base of the 

 ophthalmopod. 



The first pair of antennas approximates to that of Penseus in its general form and in 

 the arrangement of the flagella, but the prosartema is wanting, and in this respect it 

 approaches Aristeus, to which genus also it seems allied by the form of the second pair 

 of antenna? and the oral appendages. 



The three anterior pairs of pereiopoda are moderately robust, and the two posterior 

 pairs are long and feeble, and probably of little use as organs of active locomotion. 



The pleopoda are long and powerful organs and resemble those of Aristeus rather 

 than of Penseus, and the rhipidura is well developed and comparatively large. 



Geographical Distribution. — The genus evidently consists of species of natatorial 

 habit, chiefly inhabiting the depths of the sea, although its closely allied congener, 

 Gennadas, has been captured in towing nets within 500 fathoms of the surface. 



Females of Benthesicymus crenatus were taken in the Pacific Ocean near the 

 Marquesas Islands and the Low Archipelago, at a depth of about three miles. Female 

 specimens of Benthesicymus altus were obtained near Torres Strait at little under two miles 

 from the surface ; but those that I believe to be the males of this species were taken in 

 the Pacific in about 500 fathoms, off the Kermadec Islands ; north of New Zealand ; south 

 of the Celebes, and off the southern shores of Japan. A male specimen of a species differ- 

 ing from the preceding in minute details, that I have named Benthesicymus brasiliensis, 

 is recorded from the South-Western Atlantic at a depth of 1900 fathoms, with a 

 bottom temperature of 33°'l Fahr., or only 1°"1 above freezing point, whde a near 

 variety was taken at 2440 fathoms, or about three miles from the surface of the ocean, 

 off the north-western extremity of the Australian continent, and off the western side of 

 New Zealand at 1100 fathoms, and another amongst the narrow channels of the 

 Philippine Islands at 1050 fathoms, and again off the Fiji Islands in about 315 

 fathoms. 



Specimens of Benthesicymus pleocanthus were taken at 1050 fathoms, off the northern 

 extremity of the Philippine Islands, and also in the same parallel of latitude in the 



