(sensu Regan, 1909; Starks, 1910; and Fraser- 

 Brunner, 1950). The subfamily Scombiinae of 

 Frascr-Bninner, which includes all Scombridae ex- 

 cept Gastprochisma, is divisible into two major groups 

 (Collette and Gibbs, 19G3). The more primitive 

 Scomber, Rastrelliger, Scomber omor us, Grammatorcy- 

 7n(.s, and Acanthocybium have a posterior notch in the 

 hypural plate and lack a bony lateral keel on the 

 caudal vertebrae. The more advanced group, con- 

 sisting of <7;/m«o.san/a, Orcynop.tis, Sardn, Cybiosarda, 

 Auxis, Euthynnus, Katsuwonus, Allothunrms, and 

 Thunnus, lack a hypural notch and have a bony 

 caudal keel. Within the latter group another cate- 

 gory may be recognized as including Allothunnus, 

 Auxis, Euthynnus, Kaisnwonus, and Thunnus (the 

 Plcco.stei of Kishinouye, 1917, 1923), characterized 

 by the presence of well-developed prootic pits and 

 (except Allothunnus) a subcutaneous vascular sys- 

 tem. Within this group of higher scombrids, the 

 genus Thunnus is characterized by the presence of 

 fronto-parietal foramina, a particularly well-devel- 

 oped subcutaneous vascular system with two long 

 lateral branches on each side, and the body fully 

 covered with scales. Auxis and Allothunnus lack 

 fronto-parietal foramina. Auxis, Euthynnus, and 

 Katsuwonus have the body squamation limited to an 

 anterior corselet and do not have the subcutaneous 

 vascular system as well-developed &s in Thunnus: 

 the lower lateral branch is either short, or, if long as 

 in K. pelamis, it meets the upper branch mesial to 

 the ribs. 



Validity of Nominal Genera 



Cuvier (1817: 312-314) was the first to divide the 

 large Linnaean genus Scomber. For the tunas he 

 proposed Thynnus for T. thynnus and Orcynus for 

 T. alalunga. Later (in Cuvier and Valenciennes, 

 1831) he placed hissubgenusOrr^nusin thesynonymy 

 of Thynnus. Several subsequent workers independ- 

 ently realized that Thynnus Cuvier was preoccupied 

 by Thynnus Fabricius in insects. Thus Cooper 

 (1863) accepted Gill's (1861) Orycnus, a misspelling 

 of Orcynus, as a replacement name for Thynnus 

 Cuvier (see also Gill, 1889). Jordan (1888) over- 

 looked this action and proposed Albacora to replace 

 Thynnus, and Germo to replace Orcynus. Gill (1894) 

 settled matters by showing that South (184.')) had 

 previously suggested Thunnus to replace Thynnus 

 Cuvier. Most subseciuent workers have used Thun- 

 nus South either for T. thynnus alone or for several 

 or all of the seven species we refer to Thunnus. 



Whitley (1955) recently discovered an earlier modifi- 

 cation of Thynnus Cuvier, namely Thinnus S.D.W., [ 

 1837. S.D.W. (perhaps S. D. Wood, according to j 

 Whitley) emended a numb(>r of names by changing 

 y to i, ph to f, . . . . As far as we can determine, only 

 Abe (1955) followed Whitley in the usage of Thinnus i 

 S. D. W. In order to stabilize the consistent usage 

 of Thunnus South from about 1890 to the present, 

 we have applied to tlie International Commission of 

 Zoological X'^omcnclature to suppress Thinnus 

 S. D. W. (Collette and Gibbs, 1904). 



Other nominal genera have been proposed, based | 

 on anatomical data. Kishinouye (1923) described 

 two additional genera: Parathunnus based on T. 

 obesus (as mebachi) and Neolhunnus which included 

 albacares (as nuirropterus) and longyol (as rarus). He 

 based this division on anatomical characters such as 

 liver striations, the level at which the subcutaneous 

 blood vessels pass through the myomere, and pres- 

 ence or absence of the postcardinal vein. Jordan 

 and Hubbs (1925) then proposed Kishinoella for T. 

 tonggol (as raru.s), the only tuna that generallj' lacks 

 a swim bladder. We have summarized these differ- 

 ences and others that have been used to distinguish 

 genera or subgenera (table 4). A large number of 

 different arrangements can be made depending on 

 which characters one wishes to emphasize as "basic." 

 Thunnus can be divided into two groups using the 

 area of origin of the cutaneous artery, the level at 

 which it passes between the ribs, and the inter- 

 muscular bones between which it divides : T. alalunga, 

 T. maccoyii, and T. thynnus in one and T. obesus, 

 T. albacares, T. tonggol, and T. atlanticus in the other. 

 On the basis of number of arteriolar rows, T. 

 thynnus, T. maccoyii, and T. oljcsus stand out from 

 the other species. The presence of liver striations 

 and vascular cones and the length of the liver lobes 

 place T. alalunga, T. maccoyii, T. thynnus, and T. 

 obesus in one group, the remaining three species in 

 another. The absence of a swimbladder distin- 

 guishes T. tonggol from the other species, but the 

 swimbladder may be rudimentary in T. ?naccoyii, 

 and a swimbladder has been observed in small T. 

 tonggol. T. atlanticus is unique in Thunnus in hav- 

 ing 19 instead of 18 precaudal vertebrae. T. atlanti- 

 cus and T. tonggol fall together on the basis of their 

 low number of gill rakers, and T. thynnus stands out 

 with the highest number in the genus. T. alalunga 

 is uniciue in the position of spleen and stomach, in 

 the shape of the first ventrally directed parapophysis, 



98 



U.S. FISH AND WILDLIFE .SEUVICE 



