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



resemble one another so closely in form that their distinction may be most easily 

 determined by the length of the rostrum and the general robust character of those of 

 Platybema when compared with those of Latreutes. 



The branchial arrangement consists of six plumes, of which five are pleurobranchise 

 and one podobranchia, attached to the coxa of the second pair of gnathopoda, as shown 

 in the annexed table : — 



Pleurobranchiae, . . 1 1 1 1 1 



Arthrobranckise, . . 



Podobranchia?, . . . ■•■ 1 



Mastigobranchia?, . . 



in 



Geographical Distribution. — Latreutes ensiferus is abundant among the common 

 floating Gulf- weed, Sargassum bacciferum, in the Atlantic Ocean, and therefore lives at 

 the surface of the sea, whereas Latreutes dorsalis is common near Japan on a shelly 

 bottom at the depth of 8 fathoms ; Latreutes unidentatus and Latreutes planus were 

 taken near the surface at the Philippine Islands, but were not apparently very 

 abundant. 



Observations. — This genus was established by Stimpson to receive a species which 

 was first described by Milne-Edwards from a specimen taken in the North Atlantic Ocean 

 near the Azores, and which he named Hippolyte ensiferus. 1 Dr. Stimpson considered 

 it to belong to the same genus as his Latreutes dorsalis, which he found common 

 on a shelly bottom at the depth of 8 fathoms in the Gulf of Hakodadi, Japan, and says 

 that it is in close affinity with Cyelorhynchus (Platybema) of de Haan, which was taken 

 in the same locality. Close comparison of the figures of the parts given by de Haan 

 under the name of Lysmata planirostris 2 and of the figure under the name of 

 Hippolyte planirostris, z supports the opinion of the close affinity of de Haan's genus 

 Cyelorhynchus (Platybema.) with that of Latreutes, Stimpson. 



De Haan describes his genus as having the " Rostrum orbiculatum," but this cannot 

 be accepted as of generic value, inasmuch as the length of the rostrum is liable to vary 

 in the same genus, consequently its orbicular condition would become oval as it is in 

 Platybema mucronatvs (Stimpson). In a species brought from Australia, by Mr 

 Angas, and preserved in the British Museum, which I described in 1863 4 under the 

 name of Caradina truncifrons, there is little to define it from Platybema 

 planirostris (de Haan), excepting that the rostrum, instead of being orbicular, has the 

 upper distal extremity quadrate, and the carpos of the second pair of pereiopoda is 

 triarticulate. 



The two genera possess the peculiar feature in the first pair of pereiopoda of the 



1 Hist. Nat. Crust., torn ii. p. 374. 2 In v. Siebolcl's Fauna Japonica, tab. 0. 



3 Loc. cit., tab. xlv. fig. 7. 4 Trans. Zool. Soc. Lond., p. 499, pi. xl. fig. 2, 1863. 



