REPORT ON THE CRUSTACEA MACRURA. 311 



The pleopoda are long and two-branched, the inner branch of the first pair in thi 

 male being developed into a membranous petasma, while in the female it is reduced to a 

 rudimentary condition. 



The posterior pair is differentiated to form the outer plates of the rhipidura, and 

 are long and ovate ; the inner plate is strengthened by a double longitudinal median 

 rib, and the outer by a similar rib that traverses the plate subcentrally, and another 

 that lies along the outer margin, with which the median one coalesces at the margin, 

 a short distance from the distal extremity, to form a lateral tooth, near which is an 

 obsolete diaeresis. 



Aristeus differs from Penaeus in many essential points. Those which are external 

 and obvious, and can be readily used for the determination of the genus, are the first 

 pair of antennae, which have flagella unequal in length and different in form, and possess 

 the notable feature that one springs from near the base of the third joint of the 

 peduncle and the other from its distal extremity ; the absence or rudimentary condition 

 of the prosartema ; the one-jointed character of the ophthalmopod, which bears on the 

 inner side a small tubercular process which I believe can be demonstrated, in Benthe- 

 sicymus, to be a complementary eye ; the altered character of the appendage attached 

 to the mandible, which in this species is comparatively small ; certain differences 

 in the form of the oral appendages ; and, finally, the structure and arrangement of the 

 branchial plumes. 



Geographical Distribution. — This genus consists mostly of deep-water species, which 

 swim freely in the sea, and during the cruise of the Challenger were never captured in less 

 than 255 fathoms of water. 



Penseus antennatus, Eisso, the type of Duvernoy's genus, was taken in very deep 

 water in the Mediterranean, where it has since been procured by Mr. James Yates 

 Johnson, who obtained his specimens of Fnnchalia woodwardi at Madeira. The form 

 nearest to the type that I have examined is Aristem armatus, which was captured at 

 seven different localities, at depths ranging from 1400 to 2350 fathoms. The average 

 temperature of its habitat was about 36°, the highest being 38°-8, at a depth of 2050 

 fathoms, off the Philippines. Thus, this species lives in an Arctic temperature, and under 

 the pressure of a column of water more than two miles in depth, between the latitudes of 

 35° north and 35° south of the Equator. 



Running down the eastern coast of South America, in the month of September 1873, 

 the Challenger must have passed through a great multitude of young animals of this 

 genus, varying in size from 4 to 14 mm., all of which bore evidence of belonging to allied 

 species. The specimens corresponded closely excepting in such features as may be 

 dependent upon age. 



