448 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxxv. 



that the posterior margin is only half the width of the anterior. 

 Fourth segment nearly as long as wide, and considerably swollen 

 between the bases of the fourth legs. 



Genital segment flask-shaped, considerably narrowed anteriorly, 

 with well-rounded posterior corners. The latter are each armed 

 on the dorsal surface with four large triangular spines like those in 

 caudatus. On the ventral surface they show on either side two large 

 spines similar to those on the dorsal surface, and a small but distinct 

 rudimentary leg, tipped with three setae. The egg cases are very 

 short in all the available specimens, not reaching the end of the 

 abdomen, but are fully as wide as the latter and each contains from 

 fifteen to twenty eggs. 



The abdomen is elongated, about one-third the width of the genital 

 segment, and five times as long as wide; it is two-jointed, the joints 

 being of the same length; the anal laminae are long and narrow and 

 each is tipped with three seta 3 . 



Of the appendages the first antennas are long and slender, the two 

 joints of the same length, but the terminal one much the narrower; 

 both joints are well armed with setae. The second antennae are also 

 long and slender, the basal joint armed with a stout spine on its 

 posterior margin, the terminal claw bent at about its center, and car- 

 rying a small accessory spine on its posterior side near the base. 



The first maxillae have an enlarged and rounded base and a nar- 

 row terminal claw, bent abruptly near the center and more than half 

 the length of the second antennae. The second maxillae are quite dif- 

 ferent from those of exilis or caudatus; in the former the endopod is 

 simple to the very tip, in the latter it is slightly bifurcated, the in- 

 ner branch being much smaller and shorter than the outer. Here in 

 tenuifurcatus it is cut nearly to the center, the two branches being the 

 the same length. As in the other two species, these maxilla' project 

 far beyond the tip of the mouth tube; the basal joint is armed with a 

 small papilla carrying two setae, which represents the exopod. 



The mouth tube is similar to that in caudatus and exilis, but a little 

 longer, the portion beyond the hinge being longer than the basal 

 portion. The mandibles are stout, narrowed at the very tips, and 

 curved in toward each other, with fine saw teeth along their inner 

 margins. The furca is ovate, with the broad end outward, the sides 

 being slightly reentrant between the base and the arms; the latter 

 are of medium width, strongly flattened, and curve in toward each 

 other at the tips which are bluntly rounded. 



First maxillipeds of the usual pattern, the outer terminal claw less 

 than half the length of the inner one. Second maxillipeds very weak, 

 the terminal claw about half the length of the basal joint, slender, 

 a in I carrying an accessory spine on its inner margin near the tip. 

 Swimming legs of the usual pattern in this genus; endopod of the 



