SCOMBROIDS. 
91 
Lutken has proved that the characters on which Cu- 
vier’s Thynnus brachypterus is based, belong in all 
probability, simply to juvenile forms of the large 
Tunny ( Orcynus tliynnus ) and that the same author’s 
Th. coretta from the West Indies is only an inter- 
mediate form between them. To this same species 
he has also referred Stoker’s Th. secundo- dor satis 
from North America and Sciilegel’s Th. orientalis 
from Japan. Furthermore, he has shown that in 
Eutliynnus allitteratus the depth of the body and the 
height of the dorsal and anal fins eventually increases, 
and that Cuvier’s Th. hrevipinnis should be regarded 
as a juvenile form of this species. Finally, he has 
referred to the same changes of age and collected 
into one species, which he proposes to call by Lace- 
pede’s name Orcynus germo, all the long-tinned Tunnies 
known as Thynnus alalonga, Th. albicora, Th. germo , 
Th. pacificus, Th. argentivittatus, Th. balteatus, Th. sibi 
and Th. macropterus. 
Merely from these remarks we may conclude 
that the Scombroids with large jaw-teeth and with the 
dorsal and anal tins low, only slightly pointed and 
broken up into only few tinlets, may generally be re- 
garded as less advanced in development of form. From 
the measurements taken by McCoy® of different-sized 
specimens of Orcynus thynnus from 18 in. 3 lines to 
5 ft. 1 in. in length, we have the following results: 
the positions of the dorsal, anal, and ventral fins move 
forward as the fish grows older, and the height of 
the second dorsal and the anal increases, while the 
length of the pectoral fins and the snout seems first 
to increase and then relatively to decrease. The rela- 
tive length of the head, the dorsal fins, and the anal 
fin seems also to diminish, though irregularly; and as 
usual the relative size of the eyes decreases. In the 
common Mackerel, on the other hand, we find that the 
position of the posterior dorsal fin in relation to the 
length of the body moves backward as the fish grows 
older, while the distance between the front points of the 
two fins increases. We may thus distinguish between two 
different directions of development, the one represented 
by the true Mackerel, the other by the Tunnies. From 
the latter fishes Cuvier separated the genus Sarda 1 ', 
which is characterized by the comparatively large size 
of its jaw -teeth and in this respect is an intermediate 
form between the genus Scomberomorus ( Cybium ) and 
the other members of the family. Its chief peculiarity 
is that, like several species of the genus Scombero- 
morus , it displays least alteration in the extent and 
shape of the first dorsal fin from those of youth. As 
different directions of development the genera Echeneis 
and Elacate are also easy to distinguish, the former 
having the first dorsal fin changed into a sucking disk, 
the latter having this fin broken up into free spines. 
In the Scandinavian fauna only the three first- men- 
tioned branches of development a, re represented, each 
by two or three species. The systematic relations of 
these species to each other may most easily be ex- 
pressed in the following scheme: 
I: Preorbital bone comparatively nar- 
row, leaving at least the greater 
part of the upper jaw-bone exter- 
nally visible throughout its length, 
when the mouth is closed: 
A: First dorsal fin anteriorly pointed, 
with a flap-like point in front. 
Jaw-teeth comparatively small. 
Height of the anal fin greater 
O o 
than its length. Genus Or- 
cynus: 
a: Hind portion of the body 
and the belly for the most 
part naked (without distinct 
scales). Subgenus Euthyn- 
nus: 
a: Ventral sides silvery and 
either plain or spotted with 
black Eutliynnus allitteratus. 
(3: Ventral sides silvery and 
marked with black longi- 
tudinal bands Eutliynnus pelamis. 
b: Body covered with scales pos- 
teriorly and on the belly as 
well. Subgenus Orcynus Orcynus tliynnus. 
B: The anterior part of the first dor- 
sal fin forms an even, convex or 
slightly concave curve continued 
by the posterior part of the 
upper margin of the fin. Jaw- 
teeth comparatively large . Height 
of the anal fin less than its length. 
Genus Sarda: 
a: Hind portion of the body 
and the belly for the most 
part naked. Subgenus Orcy- 
nopsis Orcynopsis unicolor. 
a Prodr. Zool. Viet ., dec. V, p. 22. 
b Reyn. Anim., ed. 2, p. 199 = Pelamys, Cuv., Vai,., Hist. Nat. Poiss., vol. VIII, p. 149. 
