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



rhipidura a feature of importance in his description of species, and thus differs from 

 Milne-Edwards, who has not even taken notice either of its presence or its absence in 

 his typical species, Sergestes atlanticus. It appears to me that this tooth varies consider- 

 ably in position according to the age of the specimen, and although in some species it 

 may be either absent or more or less strongly developed in the adult, yet it certainly 

 appears to travel towards the apex with advancing age, a change produced by the 

 relatively greater growth of the appendage taking place at the base with each successive 

 moult, so that with every exuviation the tooth gets more distant, and consequently 

 appears to be nearer the apex. 



North of the Sandwich Islands, in Mid Pacific, a specimen, 20 mm. in length, was 

 taken that approaches very closely to this species, but it is much larger. It agrees 

 closely with the figure given in PL LXXI V. fig. 3, but there is a very minute denticle 

 on the rostrum which is not present in the typical specimen. 



A specimen taken off Sydney, Wellington, Australia, 12 mm. in length, corre- 

 sponds in almost every detail with the Atlantic form, so that it is impossible to 

 separate them specifically, although the localities are so distant. In this specimen the 

 little tooth which arms the outer plate of the rhipidura, is present on the right side and 

 absent on the left, which consequently diminishes the interval between this form and 

 Sergestes oculatus. 



The young of this genus are frequently, if not always, tuberculated and spinous, 

 while the adults are more generally smooth than denticulated, a circumstance that 

 might suggest that Sergestes ovatoculus and Sergestes j^trvidens were immature forms 

 of some one species ; but when we consider their near approach in relative dimensions, 

 I think I am, in the present state of our knowledge of the genus, justified in consider- 

 ing them as specifically distinct. 



Sergestes corniculum, Krciyer (PI. LXXV. fig. 1). 



Sergestes corniculum, Krciyer, Monograph. Fremstilling af Krseb. Sergestes, pp. 36, 62, Tab. iii. 

 fig. 4, a-e. 



" Eostrum projecting forwards, very slender and acute. 



" Ophthalrnopoda subequal to or a little shorter than the first joint of the peduncle of 

 the first pair of antennae, clavate ; ophthalmus very distinct from the peduncle, and about 

 one-third or one-fourth of it in length ; twice as broad as long, but the breadth not 

 reaching one-half the length of the ophthalmopod. 



" First pair of antennae with the peduncle about one-fifth shorter than the carapace ; 

 the first joint is as long as the second and third combined. 



' The second pair of antennae with the last joint of the peduncle linear, and about 

 one-third the length of the scaphocerite. 



