Carcharhinus longimanus — HUBBS 
79 
Fig. 1. Adult female of Carcharhinus longimanus (Poey), from the east-central Pacific, official photograph, 
u. S. NAVY. 
young appear to be deeper and more strongly 
compressed than the adult”; Wakiya (1924: 
232, pi. 38, fig. 2), however, indicated that 
the young of ''Naucrales indicus' are more 
slender than the adult. The original figure of 
N. polysarcus suggests that the extreme depth 
ascribed to the type may be attributed to its 
poor preservation. The different scale count, 
also emphasized by Fowler, is probably not 
significant, for the scales are too irregularly 
arranged to be counted precisely. Jordan and 
Starks (1907: 72) found that the supposed 
characters do not hold for a Pacific species, 
and Meek and Hildebrand, though failing to 
include polysarcus in their synonymy, refer 
Pacific material to N. ductor. Walford (1937: 
66-68) likewise referred the Pacific form to 
N. ductor (and by mistake labeled the figure, 
which was taken from Jordan and Evermann, 
as "from Jordan and McGregor, 1898”). An 
adult specimen 325 mm. in standard length, 
recently received from off the Pacific coast of 
Mexico, has the slender form and the short 
fins ascribed by Fowler to the Atlantic form. 
Recently, Fowler (1944: 423, 500) treated 
polysarcus as a subspecies of ductor. 
From the shark there was taken a specimen, 
105 mm. long, of another circumtropical 
species. Remora remora (Linnaeus). It agrees 
satisfactorily with material from southern 
California. 
The specific integrity of such circum- 
tropical pelagic fishes poses a problem in 
speciation, since widespread littoral types of 
the tropics, as well as most pantemperate 
types, in contrast, are ordinarily differentiated 
into allopatric subspecies or species, though 
their period of isolation has ordinarily been 
no longer, often briefer, than that of the land- 
separated populations of the tropical pelagic 
forms. The explanation seems to lie both in 
the uniformity of the tropical pelagic en- 
vironment and in the relationship between the 
population structure and the rate of specia- 
tion. Large, widespread populations that are 
not disrupted into effective reproductive units 
