Frigate Mackerel — M atsumoto 
175 
margin of body. The band more or less uniform 
in width to above tip of pectorals whence it 
narrows posteriorly to base of second dorsal fin- 
let. Body below lateral line silvery white and 
free of markings or spots. 
DISCUSSION 
Several distinguishing characters are noted in 
the two types of Auxis described here. The most 
obvious of these is the corselet formation along 
the lateral line. In thazard the corselet narrows 
abruptly toward the lateral line and ends at 
about two-thirds of the distance from the origin 
of the first dorsal to the origin of the second 
dorsal fin. On larger specimens, 350 to 400 mm. 
long, according to Wade (1949: 234), the 
corselet tapers less rapidly than in smaller speci- 
mens, but in no case does it extend to the level 
of the second dorsal origin. In contrast to this 
the corselet of thynnoides tapers very gradually 
and terminates between the first and second 
dorsal finlet. In the Hawaiian specimens the 
corselet is still 16 to 18 scale-rows wide beneath 
the origin of the second dorsal fin. In the smaller 
Philippine specimens, whose total lengths range 
between 195 and 235 mm., the corselet beneath 
the second dorsal fin is from 9 to 15 scale-rows 
wide. 
Upon examining the corselet of these speci- 
mens, it was noticed that the largest scales were 
adjacent to the lateral line, and that the size of 
the scales diminished on each succeeding scale- 
row away from the lateral line. From this ob- 
servation and also from the previous one, that 
the larger Hawaiian specimens have more scale- 
rows than the smaller Philippine specimens, it 
was suspected that the width of the corselet 
increased with growth of the fish. An analysis of 
the regression of number of scale-rows on fish 
length ( Hawaiian and Philippine material com- 
bined) yielded the equation, Y = 0.078X-4.55, 
which is shown graphically in Figure 3. The 
highly significant regression coefficient (b = 
0.078; t =: 12.14, P < 0.001, at 19 degrees of 
freedom) indicates that the number of scale- 
rows increases as the body length does, and 
that therefore, the variation in corselet width 
between the Philippine and Hawaiian speci- 
mens represents growth rather than a species 
difference. 
An interesting character, one which has sel- 
dom been used previously in separating the two 
forms of Auxis, is the configuration of body 
markings. In both species the dorsal half of the 
body contains a number of dark bars. Although 
these bars vary in number and are sometimes 
broken into spots, their position on the body is 
distinctive. In thazard the bars lie obliquely in 
a posterodorsal direction. In addition to these 
bars, the dorsal surface of the body is marked 
with a dark longitudinal band which is as wide 
as the interorbital space and which extends 
evenly from the snout to above the tip of the 
pectoral fin. It then tapers gradually to about 
the base of the second dorsal finlet. In thyn- 
noides, on the other hand, the bars lie in a more 
Fig. 2. Auxis thazard (Lacepede). 
