HALIPORUS THETIS. 193 



a low keel. The inferior keel gives off a branch from its lower or external 

 side, near the posterior border of the carapace. Tliere is, besides, an inferior 

 submarginal keel running from the bi'anchiostegal spine back to the hind 

 border of the carapace. The several fields or areas included between the 

 grooves and carinas above mentioned are more or less rough with corrugations 

 of the integument. 



All the segments of the abdomen are carinated along the median dorsal 

 line. Their pleurae are shallow and rounded. On each side of the segments 

 is seen a longitudinal furrow, whose edges tend to rise into carinoe. Tliis 

 furrow is most emphatic on the fifth and sixth segments. The sixth seg- 

 ment is once and a quarter as long as the fifth segment, but only two 

 thirds the length of the telson, which is armed with a pair of conspicuous 

 lateral spines near the tip and about four pairs of verj minute spinules on 

 the margin in front of them. 



The ophthalmic peduncles are one half as long as the rostrum. On their 

 inner side is borne a small tubercle. The eyes are large, black, broader than 

 their peduncles. The basal segment of the antennule is armed on the ex- 

 ternal margin with two small spines, one near the middle, the other at the 

 distal end ; the second segment is nearly twice the length of the succeeding 

 one, hairy, with both the inner and outer edges of its upper surface raised 

 into visible carinae ; the flagella are lost in the single specimen examined. 

 The basal segment of the antenna is equipped with an external spine 

 directed- downward and forward ; the scale is longer than the anteunular 

 peduncle, rather broad, thin and membranaceous in texture. The tips of 

 the third maxillipeds when extended forward reach beyond the distal end 

 of the antennal scale ; the carpus joint is expanded, and flat on its inner sur- 

 face. The third pair of legs reach forward about as far as the third maxil- 

 lipeds. The last pair of legs, although rather short for the genus, reach 

 forward beyond the distal ends of the third pair ; the merus and carpus are 

 of equal length, the propodite half the length of the preceding segment, and 

 curved. The dactylus is one half as long as the propodite. The exopods 

 of the third maxillipeds are small, and throughout the series of legs these 

 organs are so small that they may be considered rudimentary. There are 

 rudimentary podobranchial plumes, discernible by the aid of a lens, attached 

 to the epipods of the third maxillipeds and the first three pairs of legs.* 



* Accordiii!^ to Spence Bate, there are rudimentary brancliial plumes attached to the epipods of the 

 third maxillipeds and the first pair of legs in the genus Ihdiporus. In H. nereits and H. doris (as in Eyme- 



