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



The terminal pair, which helps to form the rhipidura, is a little longer than the telson, 

 and has a diaeresis armed on the outer margin with a well-defined tooth. 



Observation. — It is interesting to notice the close approximation of this species to 

 Palsemon squilla, the habitats being the antipodes of each other. 



Palsemon natator, Milne-Edwards (PI. CXXVIII. figs. 6, 7). 



Palsemon natator, Milne-Edwards, Hist. Nat. Crust., torn. ii. p. 393. 



Leander erraticus, Desmarest, Ann. Soc. Entom. France, torn. vii. ser. 2, p. 87, 1849. 



Leander natator, Stimpson, Proc. Acad. Nat. Sci. Philad., p. 109, 1860. 



Male. — Carapace about one-third the length of the animal and anteriorly produced 

 to a rostrum that is subequal in length to the carapace, and armed on the upper margin 

 with nine or ten teeth and on the lower with six ; frontal margin armed with an antennal 

 and a branchiostegal tooth. 



The pleon has the third somite longer than the others, dorsally arcuate, and pro- 

 jecting posteriorly in the median line over the succeeding somite. 



The ophthalmopoda are robust and largely project beyond the pedicular attachment ; 

 the ophthalmus is hemispherical, and is furnished with a distinct ocellus on the posterior 

 surface, in contact with the margin. 



The first pair of antennas has the flagella long and slender, the upper and outer 

 being nearly as long as the animal, and the inner and lower about two-thirds the length 

 of the outer ; the third or secondary branch of the outer is about half the length of the 

 inner, much stouter than either of the two, and continues the same almost to the 

 extremity ; the first joint of the peduncle is longer than the second and third together, 

 it is broadly dilated, of great tenuity, and armed with a stylocerite that reaches to half 

 its length, and a sharp-pointed tooth on the outer distal angle ; the third and fourth 

 joints are cylindrical and short. 



The second pair of antennas is as long as the animal and has the flagellum as slender 

 as those of the upper ; it carries a scaphocerite that is equal in length to the rostrum 

 or carapace, and armed on the outer distal angle with a sharp tooth, whence the anterior 

 foliaceous margin advances obliquely forwards beyond the apex of the external tooth ; 

 the inner margin is subparallel with the outer, and the basal joint is armed with a 

 strong, sharp tooth near the base of the scaphocerite. 



The mandible resembles that of Palssmon serratus in character, but has the molar 

 process more strongly dentate and the psalistoma broader and strongly serrate ; the 

 synaphipod is shorter than the psalistoma and of extreme tenuity. 



The succeeding oral appendages are of generic value only, and closely resemble those 

 of Palsemon serratus. 



The first pair of gnathopoda carries a very rudimentary mastigobranchial appendage 

 and a small podobranchial plume. 



