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



The surface of the shell is smooth, and faintly marked out into polygonal areolae, the line 

 of junction of the valves on the dorsal surface is depressed in front of and behind the 

 middle, while the contact margins on the ventral surface are produced into a longitudinal 

 keel. Length, l-55th of an inch ('44 mm.). 



This species, which, probably, when the anatomical structure is known, will have to 

 be made the type of a new genus, was found in a dredging from the Straits of Magellan, 

 lat. 52° 20' S., long. 68° 0' W., depth 55 fathoms (Station 313). 



[PI. VI. fig. 7, a-d. a Shell seen from left side, h from above, c from below, d from 

 front. Magnified 80 diameters.] 



Pseudocy there, G. 0. Sars (1865). 



Shell thin, pellucid, compressed, rounded in front, produced behind; hinge-joint 

 simple. Anterior antennae bearing several long setae, seven-jointed ; second joint stout, 

 having a single seta on the middle of the anterior border, last joint narrow, long, and 

 bearing very long apical set^ ; posterior antennae very slender, five-jointed, flagellum 

 long and slender. Mandibles small, with slender curved claw-like teeth, palp narrow, 

 four-jointed ; branchial appendage bearing long setae. Terminal lobes of the first pair of 

 jaws narrow, and having a large elongated ovate branchial plate, which is provided at 

 the base with three curved and deflexed setae. Feet very long and slender ; abdomen 

 ending in a long slender process ; postabdominal lobes narrow and setiferous. No eye. 



Though diff'ering from Bythocythere to some small extent in anatomical details, this 

 genus is perhaps more distinctly characterised by the structure of the shell, which is 

 extremely thin and delicate, nearly or quite devoid of sculptured ornament or definite 

 microscopic structure, and very much compressed. As regards the species at present 

 known to us, there is no difiiculty in locating accurately the members of these two genera, 

 but they approach each other so closely that if, as is most likely, new and intermediate 

 sj)ecies come to light, it may become impracticable to maintain the separation. 



The genus Pseudocythere is widely distributed, occurring in the European Seas as well 

 as in those distant regions of the Southern Hemisphere here noted. 



As a fossil it has been recognised only in the Post-Tertiary deposits of the British Islands. 



1. Pseudocythere caiidata, G. 0. Sars (PI. I. fig. 6, a-d). 



Pseudocythere caudata, Sars, Oversigt Norges marine Ostrac, p. 88. 



Pseudocythere caudata, Brady, Monog. Eeoent Brit. Ostrac, p. 453, pi. xxxiv. figs. 49-52, 



pi. xli. fig. 6. 

 Pseudocythei-e caudata, Brady, Crosskey, and Eobertson, Post-Tertiary Entomostraca, p. 210, 



pi. ii. fig. 9. 



Carapace compressed, elongated ; seen from the side, oblong, subquadrate, scarcely 

 higher in front than behind ; height equal to half the length ; anterior extremity evenly 



