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



at the meros. The second pair is long and slender, fringed with long hairs, and 

 terminates in a small but well-formed chela. The third pair is wanting, being broken 

 off at the basis. The fourth pair is broken off at the middle of the carpos, but, judging 

 from the portion left, it is a longer appendage than usual ; the fifth pair is short, 

 cylindrical, and saccular, and within the sac three articulating divisions are apparent. 



The pleopoda are long and slender, the first pair being single-branched, the others 

 unequally biramose, and becoming slightly more robust posteriorly. Those of the sixth 

 pair help to form the rhipidura, and are longer than the telson, the outer branch being 

 furnished with a tooth that is one-third distant from the distal extremity. 



Length, 10 mm. (0'4 in.). 



Habitat. — April 26, 1876 ; St. Vincent, Cape Verde Islands; at the surface. 



Observations. — This species differs from Sergestes nasidentatus in having the small 

 tooth on the dorsal crest a little posterior to the frontal margin, a cusp on the gastric 

 region, and a small tooth at the posterior dorsal extremity of each of the three posterior 

 somites of the pleon. 1 



Sciacaris, 2 n. gen. 



Like Sergestes,but having the telson terminating in two lateral uniarticulate appendages. 



This genus, if genus it be, is founded on three specimens in three different stages. 

 The youngest is in the Acanthosoma stage, the second in the Mastigopus condition, and 

 the third in that of the young adult. They appear to be different stages of two species, 

 and as such I shall describe them. 



The several specimens were taken off New Guinea, and in the North Pacific Ocean. 



Sciacaris telsonis, n. sp. (PI. LXXVIII. fig. 1). 



Carapace nearly half the length of the animal, not including the rostrum or telson. 

 Eostrum long, slender, and nearly half the length of the carapace. Frontal margin 

 armed with a slender tooth on the outer side of the first pair of antennae. Dorsal surface 

 and lateral margins smooth. 



Pleon having the first five somites subequal, and the sixth about as long as the three 

 preceding united. 



Telson (fig. lz) about half the length of the sixth somite, bifurcate at the extremity, 

 each process supporting a small articulating joint of considerable tenuity, armed at the 

 apex and outer margin with one or two articulated spines. 



The ophthalmopoda are clavigerous and about as long as the rostrum. 



1 Since the foregoing was in type I have discovered that the genus Podopsis, established in 1829 by Vaughan 

 Thomson, is synonymous with Sergestes. But as the former name was used in 1819 for a genus of Mollusca by Lamarck, 

 Milne-Edwards' appellation must stand. 



8 From aKta., shadow, and x.a,pk, shrimp. 



