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



of the fourth pair of pereiopoda to the posterior margin of the sternum ; the inner margins 

 are brought into contact and turned downwards, and form the cleft of which de Haan 

 writes (vide fig l'", $). The oviducts open at the base of the coxae of the third pair of 

 pereiopoda by a foramen placed at the extremity of a short tubercle, the margin of 

 which is copiously fringed with hairs. The first pair of pleopoda has the outer ramus 

 long, flat, and subfoliaceous, and the inner reduced to little more than a rudimentary bud. 



In the male the two oval plates are not present, but a small sharp prominence exists 

 in the median line towards the anterior portion of the ventral surface of the posterior 

 somite. The vas deferens debouches near the base of the coxa, where that joint articulates 

 with the pereion. 



The first pair of pleopoda has the external ramus similar to that of the female, but 

 the inner branch is altogether altered from its normal form and condition. It 

 articulates with the basisal joint near its base, and is developed into a large, membranous 

 and flexible plate, longitudinally folded on itself, and united with its fellow in the median 

 line by a series of small cincinnuli, which thus form a large petasma that overlies the 

 sexual apparatus. 



The other pairs are biramose, but have the outer branch twice as long as the iuner. 



Milne-Edwards records his specimen from the coast of India, and Siebold obtained 

 that which de Haan described from Japan. 



Eatber than multiply species upon grounds of slight variation, I have thought that 

 as Penseus monodon has never been figured, and varies in some points in almost every 

 description, and as the Challenger specimens so closely resemble Penseus semisulcatus, 

 and correspond with the description given by Milne-Edwards and de Haan, they may 

 be accepted as the typical form of the species. 



Dr. Camil Heller, in his account of the " Novara " Crustacea, has described a species 

 under the name of Penseus tcdiitensis, 1 the figure of which varies but little from this species, 

 but the description does not agree with his figure. He says that the inferior margin is 

 without teeth (" margine inferiore edentulo "), but represents three teeth in his figure ; he 

 also says that the flagella reach to the last tooth on the rostrum, but figures them as 

 being as long as the carapace. If the figure be correct, the species corresponds very 

 closely with that given by Dana as Penseus carinatus, and which I think there is little 

 doubt is identical with Penseus monodon. The extremity of the rostrum is very 

 slightly turned up in both de Haan and Dana's specimens, whereas in the type according 

 to Milne-Edwards the rostrum is straight, and the Challenger specimens agree with this. 



Heller's species is 120 mm. in length, and was taken off the Island of Tahiti, while 

 Dana's Penseus carinatus was taken near Singapore. 



Penseus escidentus, Haswell, taken at Port Jackson, probably belongs to this species 

 also. 



1 Eeise der Fregatte " Novara," Crustacea, p. 121, Taf. xi. fig. 2. 



