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



minute size. The second and third pairs are long, slender, and terminate in small chelae. 

 The fourth and fifth pairs are entirely absent. 



The pleopoda are slender, not very long, and gradually become shorter and more 

 robust. 



The sixth pair of pleopoda is about as long as the sixth somite of the pleon ; the 

 outer margin of the external plate is armed with a small tooth halfway between the 

 basal and distal extremities ; the rest are furnished with long ciliated hairs. 



Length, 12 mm. (0"5 in.). 



Habitat.— Station 106, August 25, 1873; lat. 1° 47' N., long. 24° 26' W. ; Mid 

 Atlantic Ocean, within 40 fathoms of the surface. 



Station 354, May 6, 1876 ; lat. 32° 41' N., long. 36° 6' W. ; North Atlantic Ocean. 



Observations. — The specimen from the former station appears to correspond with the 

 genus Acetes in the absence of the posterior two pairs of pereiopoda, but 1 am inclined 

 to think, from the presence of these appendages in a rudimentary condition in the 

 specimen from the latter station, that their absence is owing to the early stage of 

 development. 



The telson is rather longer in the specimen from the tropical part of the Atlantic 

 than in that from the North Atlantic, which also has the ventral surface produced to large 

 lobes in the median line, the anterior two being; each furnished with a short strong tooth. 



Sergestes penerinkii, n. sp. (PL LXXVI. fig. 3). 



Carapace about one-third the length of the animal ; rostrum short, about one-fourth 

 the length of the carapace, and projecting horizontally forwards ; frontal margin not 

 produced to a tooth at the outer angle of the orbit. 



Pleon having the first two somites armed on the ventral surface in the median line 

 with a strong, sharp tooth, broad at the base, and directed forwards ; the second being- 

 more prominent than the first. The third, fourth, and fifth somites are dorsally armed 

 with a strong tooth in the median line of the posterior margin, and on the ventral surface 

 in the median line with a strongly projecting unarmed lobe. The sixth somite is furnished 

 with a tooth on the posterior dorsal margin, and is about as long as the two preceding- 

 somites together. 



Telson about half the length of the sixth somite. 



The ophthalmopod is very nearly as long as the carapace, the stalk slender, and the 

 ophthalmus broad. 



The first pair of antennas having the extremity of the peduncle reaching scarcely 

 beyond the extremity of the ophthalmopod ; the three joints subequal or nearly subequal 

 in length. 



