S('OJIIi]{(HDS. 



91 



LuTKEN has pi'ovcd tliat the cliaracters on wiiicli Cu- 

 vier's TliijiniKs byacTiypteriis is i)as('(l, hflonii- in all 

 probability, sinij)!)' to jnvcnilc forms of tlic iari^e 

 Tunny (Orci/nus tlii/uiuis) and that the, same author's 

 Th. coretta from the West Indies is only an inter- 

 mediate form lietween tliem. To this same species 

 he has also referred Stoheks Th. secundo-dorsalis 

 from North America and Schlegel's TIi. orimfti/is 

 from Japan. JMirtherniore, lie has shown that in 

 Euthynnus alUtteratus the depth of the body and tlie 

 height of the dorsal and anal fins eventually increases, 

 and tliat Cuvier's TIi. 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-finned Tunnies 

 known as Thynnus alalonga, Th. albirora, Th. [lermo, 

 Th. pacificns, Tli. argentivittatus, Th. halteatus, Th. .sibi 

 and Th. macr obiter us. 



Merely from these i-emarks we may conclude 

 that the Scombroids Avith large jaw-teeth and with the 

 dorsal and anal fins low, only slightly ])ointed and 

 broken up into only few finlets, 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 j)ectoral 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 tlie l)od}- moves backAvard as the fish grows 

 older, while the distance between the front points of the 

 two fins inci'eases. We may thus distinguish between two 

 different directions of development, the one represented 

 by the true Mackerel, the other l>y the Tunnies. From 

 the latter fishes Cuvif.r separated the genus Sarda'', 

 which is characterized by the comparativelv large size 

 of its jaw-teetli and in this respect is an intermediate 



form between the genus Scomberomonis (Cyhium) and 

 the other members of the family. Its chief peculiarity 

 is that, like several species of the genus Scombero- 

 monis, it displays least alteration in the extent and 

 shape of tlie first dorsal fin from those of youth. As 

 diftei'ent directions of development the genera Echeneis 

 and Elacate are also easy to distinguish, the former 

 liaving tlie first dorsal fin changed into a sucking disk, 

 the latter having this fin broken up into free spines. 

 In the Scandinavian fauna onlv the three first-men- 

 tioned branches of development are 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: Preorhital bone compaiiitively 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 

 than its length. (ienus Or- 

 cynns: 



a: Hind portion of the body 

 and the bellv for the most 

 part naked (without distinct 

 scales). Subgenus Euthyn- 

 nus: 



a: Ventral sides silvery and 

 cither plain or spotted with 

 black Euthynnus alUtteratus. 



(5; Ventral sides silvery and 

 marked with black longi- 

 tudinal bands - Euthynnus pelamis. 



b: Body covered with scales pos- 

 teriorly and on the belly as 

 well. Subo-enus Orcynus Orrijnus tlii/nnus. 



B: The anterior part of the first dor- 

 sal fin forms an even, convex or 

 slio^htlv concave curve continued 

 by the posterior part of the 

 upper margin of the fin. Jaw- 

 teethcomparativelylarge. 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. 



" Prodr. Zool. Vict., dec. V, p. 22. 



' Begti. Aiiiiii., cd. 2. y. 109 = I'elamys, Civ., Vai... ///.-■■(. Xat. Poiss., vol. VIU. p. 149. 



