62 THE VOYAGE OF H.M.S. CHALLENGEE. 



a pleurobranchia springing from a fissure that defines a separation between the 

 somites ; the point of attachment is very low, not very much above the podopleural 

 articulation. 



The second and succeeding pereiopoda are themselves rather more slender than the 

 first pair, but this is chiefly due to the lower margin of the propodos being hollowed or 

 excavate in all but the first. The third pair in the female carries the foramen of the 

 vulva near the inferior podopleural articulation, in the form of a circular opening on a 

 prominent elevation. The fifth or last pair of pereiopoda in the male carries the foramen 

 of the vas deferens on the anteroinferior angle of the coxa, and is much larger in 

 diameter than that of the vulvar opening in a female of corresponding size. This last 

 pair of j>ereiopoda has no mastigobranchia or branchial plume, except a pleurobranchia, 

 and this is implanted very near to the podopleural articulation. 



The arrangement of the branchiae may be best seen in the following table : — 



Pleurobrancliiae, 

 Artbrobranchiae, 

 Podobranchiae, 

 Maetigobranchiffi, 



This species affords the only specimens of the genus taken elsewhere than along the 

 Pacific coasts of Asia and the Australasian Islands. Hence the similarity that it bears 

 to Ibaccus incisus (Peron) is the more remarkable, and, judging by the several figures 

 and descriptions published, the differences are slight, except in the character and number 

 of the dentations that arm the margins of the carapace and antennae. 



Ibaccus brevipes, n. sp. (PI. IX. fig. 1). 



This species resembles Ibaccus verdi. and is armed with seventeen small teeth along 

 the lateral margin of the branchial region of the carapace ; none or only a fine serrature 

 on the outer margin of the anterior angle of the carapace ; three or four along the outer 

 margin of the posterior antennal plate ; and six or seven along the distal margin of the 

 anterior antennal plate. 



The pleopoda are very small and biramose. the inner ramus supporting a com- 

 paratively large stylamblys. 



Length, 67 mm. (27 inches). 



Habitat— Station 192, September 26, 1874 ; lat. 5° 49' 15" S., long. 132° 14' 15"E.; 

 off the Ki Islands ; depth, 140 fathoms ; blue mud. 



There is but one specimen of this species in the collection, and that a male, which 

 1 Mars all the characteristics of being immature; and had it been taken anywhere near 



