40 BULLETIN 201, UNITED STATES NATIONAL MUSEUM 



in my interpretation of the facts it is evident that the fully developed 

 condition of the pleopods in the male comes very late and is coincident 

 with the attainment of sexual maturity. 



There now arises the question of the status of Petalophthalmus 

 pacificus Faxon. I have already shown ( 1925) that this species agrees 

 with P. armiger in every character except that of the pleopods, and 

 that the other points of difference pointed out by Faxon disappear on 

 examination of the Challenger type. The pleopods of P. pacificus 

 differ from those of the Challenger specimen of P. armiger only in the 

 much more rudimentary condition of the exopod, virtually absent in 

 the first pleopod and appearing as a very small rudiment on the re- 

 maining pleopods. Apart from the type only one other specimen has 

 been referred to P. pacificus, that recorded from Hawaii by Ortmann 

 (1905). The type specimen measured 31 mm., but Ortmann gives no 

 size for his specimen. Hansen (1927) suggested that the differences 

 between the pleopods of P. pacificus and P. armiger were due to the 

 immature state of Faxon's specimens. From what has emerged above 

 from the study of a really adult male there is considerable evidence 

 to show that Hansen's suggestion is correct. If the Challenger speci- 

 men is immature at 40 mm. a specimen at 31 mm. would obviously be 

 still more immature. I think there can be no doubt that P. pacificus 

 was founded on a very young male specimen of P. armiger, in which 

 the secondary sexual characters of the pleopods had hardly begun to 

 manifest themselves. I have therefore united Faxon's species with P. 

 armiger on these grounds. 



The telson of these specimens has from 40 to 50 spines on each lat- 

 eral margin and five large barbed spines on each side of the slightly 

 emarginate apex with a single median spine between the two series. 

 Between the central spine and the inner spine of the lateral series at 

 the apex there are five small spinules. The telson shows variation in 

 its armature correlated with age. I have examined two small speci- 

 mens taken off the west coast of Ireland, measuring 14 and 15 mm., in 

 which the number of spines on the lateral margins is only 10 and 13 

 respectively. The number of spines on the lateral margins increases 

 as growth proceeds. So does the number of spines at the apex. In 

 the smaller of the above mentioned specimens there are only three 

 spines on each half of the apex and the number of spinules on each 

 side of the central spine is only three. 



PETALOPHTHALMUS OCULATUS Illig 



Figure 5 



Petalophthalmus oculatus Illig, 1906a, p. 194, fig. 1 ; 1930, p. 411, figs. 20, 21.— 

 Tattebsaix, 1937, p. 1 ; 1939a, p. 229, fig. 3. 



Occurrence. — Puerto Rico : Johnson-Smithsonian expedition station 

 99, series number 573, latitude 18°40' K, longitude 64°51' W., 220 



