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



armed on the outer or smooth side near the apex with a very long and narrow tooth, 

 slightly serrate at the margins, and on the inner side with long ciliated hairs. 



The oral appendages are advanced in development. 



The third pair of siagnopoda or maxillipede is biramose, and has the several joints 

 fringed with hairs. 



The first pair of gnathopoda consists of a long basal joint, which supports two 

 branches ; that which represents the permanent limb is four-jointed, while the branch is 

 a slender, feeble appendage. 



The second pair differs from the first in being longer and in having the branch or 

 ecphysis larger. 



The five pairs of pereiopoda are all similarly developed, but become slightly shorter 

 posteriorly. 



The pleopoda exist as long, narrow, bud-like sacs, except the sixth pair, which 

 is large and well developed. Its two branches are subecpial in length, fringed with 

 long hairs, and not longer than the telson. The outer plate has the external margin 

 smooth in its basal half, where it is armed with a long and slender tooth. 



Length, 7 mm. (0"28 in.). 



Habitat. — The South Pacific Ocean. 



This form, as well as Acanthosoma brevitelsonis, should be studied in connection with 

 the younger stages of Mastigopus (PL LXV. figs. 3, 4), and compared with Sergestes 

 brachyorrhos and Sergestes caudatus, Kroyer, in which the short and long telson 

 respectively is persistent in the adult. 



Another specimen was taken in the Atlantic, on the 13th April 1876, which corre- 

 sponds in several respects with the present form, but differs in certain details that may 

 depend upon a slight difference in age, and in others of more or less importance. 



The form of the rostrum and carapace is the same as in the preceding, but the latter 

 bears a large smooth tooth on each side of the gastric region, and a small tubercle in the 

 median line between them, and just within the posterior margin ; also in the median 

 line there is another long anteriorly curved tooth. 



The pleon corresponds in its adornment with that of the previous specimen, except 

 that the posterior somite is dorsally armed with a long, serrate, spine-like tooth, and 

 each of the postero-lateral angles terminates in a short, sharp, plain tooth, instead of in 

 a long ornate one. 



The appendages correspond with those of the specimen taken in the South Pacific, 

 except that the pleopoda are a little more developed, and the outer plates of the rhipidura 

 are slightly spatuliform, and have the tooth that arms the outer margin somewhat nearer 

 to the distal extremity than to the base, and standing at the broadest diameter of the 

 plate, which is furnished distally and on the inner side with very long ciliated hairs. 



