REPORT ON THE CRUSTACEA MACRURA. 351 



The first pain of antennae is long and slender, having the peduncle long, without 

 prosartema or stylocerite, terminating in one long, slender flagellurn, and a short 

 rudimentary one, which in the male is frequently furnished with a small tertiary branch, 

 while the large flagellurn becomes bulbous at the base, and supports a fasciculus of 

 membranous cilia. 



The second pair of antennae is long and extremely slender, and carries a large 

 scaphocerite. The mandibles have the incisive margin smooth, and carry a long 

 and narrow two-jointed synaphipod ; the other oral appendages correspond to those 

 of Gennadas and Benthesicymus. 



The first pair of gnathopoda is large and very robust, being generally reflexed upon 

 itself, but not closely so, and terminates in a short blunt dactylos. The second pair is 

 long, slender, and feeble. 



The first pair of pereiopoda has only six joints, the dactylos apparently being absent. 

 The second and third pairs of pereiopoda are long and slender, terminating in minute 

 chelae. The fourth and fifth pairs are short and feeble, and terminate in simple dactyli. 

 The pleopoda are biramose, except the first pair, which carries a petasma attached to 

 the basal joint in the male and which becomes rudimentary in the female. The 

 posterior pair of pleopoda is modified to form the rhipidura, and is much longer than 

 the telson. 



This genus was first established by Professor Milne-Edwards, in November 1829, in 

 a paper read before the Academy of Sciences upon a species taken in Mid-Atlantic. 

 It has since been more fully elucidated by Kroyer in a monograph published in 

 1856, ' in which he has modified Milne-Edwards' description by showing that the 

 pereiopoda, which Milne-Edwards says are " filiformes et monodactyles," are not so. 

 The second and third pairs, when examined with a lens of low magnifying power, are 

 seen to be minutely chelate, and the others appear to be wanting in a joint ; and the 

 onathopoda have an increased resemblance to a pediform character, the first pair being 

 the most powerful of all the appendages. 



The general appearance is that of a depauperised Penseiform Crustacean, and the con- 

 dition of the branchial organs supports this idea. These consist of a single row of 

 branchial plumes, which, in accordance with the nomenclature that I have used, are all 

 pleurobranchiae, but the posterior plume, instead of being attached to the somite that 

 carries the posterior pereiopod, is attached to the penultimate somite, which consequently 

 carries two plumes. 



Thus there are seven plumes connected with six somites, which are arranged as 

 follows. The first pair of gnathopoda carries a small, circular, discoid mastigobranchial 

 plate, which calls to remembrance the form of the branchial appendage in the Amphipod 



1 Forsdg til en monographisk Fremstilling af Krcebsdyrstegten Sergestes. K. dansk. Vidensk. Selsk. Skriv., Bd. iv. 

 p. 217, Copenhagen, 1855 ; Zeitschr. d. gesammt. Natururiss, vol. viii. p. 413, Halle, 1856. 



