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



description, is very similar to the same organ in Penseus monodon, and taking all things 

 into consideration, I am induced to believe that Penseus indicus is but an overtoothed 

 variety of Penseus monodon; it is interesting as showing from what small variations 

 forms of importance may gradually proceed. 



Penseus monodon, Fabricius (PI. XXXI V. fig. 1). 



Penxus monodon, Fabricius, Suppl. Ent. Syst., p. 408. 



Penxus monodon, Milne-Edwards, Hist. Nat. Crust., torn. ii. p. 416. 



Penxus semisulcatus, de Haan, Crust, in v. Siebold, Fauna Japonica, p. 191, pi. lvi. fig. 1. 



Penxus carinatus, Dana, U.S. Explor. Exped., Crust., p. 602, pi. xl. fig. 2. 



Penxus monodon, Sp. Bate, Ann. and Mag. Nat. Hist., ser. 5, vol. viii. p. 178, pi. xi., 1881. 



Eostrum straight, dorsally elevated into a laterally compressed crest that is armed 

 with six or seven teeth, and on the lower margin with three. Rostral crest gradually 

 lessens behind the last tooth, and is lost before reaching the posterior margin of the 

 carapace ; in the male it exists in the form of a simple carina, and in the female it is 

 longitudinally grooved. On each side of the rostral crest is a sulcus, formed by a 

 longitudinal ridge that commences at the apex of the rostrum and terminates in a hue 

 corresjjonding with the posterior tooth of the dorsal crest. There is one tooth on the 

 basisal joint of the anterior two pairs of pereiopoda and none upon that of the third. 

 The telson is grooved in the median line and fringed with fine hairs on the margins. 



Length, 190 mm. (7"5 in.). 



Habitat— Station 188, September 10, 1874 ; lat. 9° 59' S., long. 139° 42' E. ; south 

 of New Guinea; depth, 28 fathoms; bottom, green mud. Two specimens; a male and a 

 female; associated with Penaeus incisipes. 



Station 203, October 31, 1874 ; lat. 11° 6' N., long. 123° 9' E. ; off Panay, Philippine 

 Islands ; depth, 20 fathoms ; bottom, mud. Two specimens ; females, associated with a 

 male of Penseus indicus ; trawled. 



The typical specimen of Fabricius of this species is not now in existence, and those 

 from which Milne-Edwards drew up the short description in his Histoire Naturelle des 

 Crustacds, are all small and immature animals, but he says that although he had 

 not seen specimens that were more than 76 mm. (3 in.) in length, yet according to 

 Fabricius they reach to a very large size. 



The examination of the specimens preserved in the Museum of the Jardin des Plantes, 

 and their comparison with others of a larger size, have convinced me that Penseus monodon 

 is identical with Penseus semisulcatus of de Haan, as figured and described by him in 

 Siebold's Fauna Japonica. De Haan rests the distinguishing feature, that separates the 

 two species, on there being a median longitudinal groove between the base of the rostrum 

 and the posterior margin of the carapace in Penseus semisidcatus, and none in Penseus 



