680 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxxi. 



Frontal plates narrow and not quite half the width of the carapace; 

 lateral lobes reaching back to the posterior margin of the first free 

 thorax segment; no eyes visible. 



Transverse grooves separating the lateral areas situated far l)ack, 

 leaving the thoracic portion shorter than the cephalic, as in caudatus. 

 These grooves do not make a prominent notch, however, at the edge 

 of the carapace, as in the other two species. But this may well be 

 the fault of the preservation of the specimen, since it has evidently 

 shrunken considerably in the alcohol. 



The first free (third) thorax segment is wide and short, while the 

 fourth is longer and narrower and subquadrilateral in outline, showing 

 no increase in width between the bases of the fourth legs. The gen- 

 ital segment is flask-shaped, but its exact proportions and size can not 

 be definitely determined, in consequence of an injury, and also because 

 it is entirely covered with Protozoa. It is certain, however, that it is 

 more than half the size of the carapace, probably fully three-fifths; 

 that the posterior corners are well rounded, and that they do not show 

 any signs of rudimentary legs or spines, as in caudatus. In this 

 respect it is similar to exilis. 



The abdomen is almost linear, nearly twice the length of the genital 

 segment and more than eight times as long as wide. 



It is jointed once at the center, the joints being thus of the same 

 length, and the terminal one bearing a pair of short and narrow anal 

 lamina?. 



The appendages are very similar to those in caudatus., the chief 

 difference being that they are more slender and comparatively longer. 

 In the second antenna? the basal joints are more slender than in either 

 of the other two species, but the long distal claw is considerably stouter, 

 being fully half as wide as the basal joints. The first maxilla^ have a 

 swollen circular base and a long terminal portion less than one-fifth 

 the width of the base and bent abruptly at a right angle near the cen- 

 ter. The furca is long and slender, with linear rami, which are nearly 

 parallel and more than twice as long as the basal portion. The foramen 

 is small and almost circular. 



The swimming legs are of the usual pattern, both rami of the fourth 

 pair being three-jointed. 



Total length 6.5 nun. Length of carapace, including the third 

 thorax segment, 2 mm.; of the fourth segment, 0.5 mm.; of the geni- 

 tal segment, 1.5 mm.; of the abdomen, 2.5 mm. Width of carapace, 

 2.4 mm. 



Color of the preserved specimen a darker yellow than in caudatus, 

 without any pigment. 



{ttnuifurcatus, tenuis, slender, \\x\A furcatus, furnished with a furca.) 



This species was founded by Rathbun in 1887 upon a single poorly 

 preserved specimen. As a usual thing under such circumstances it is 



