surface (sphcnotic head). T.cast width (A) and 

 greatest width wore measured vertically on the 

 anterior (sphenotic) head. 



Metapterygoid (See fig. 7). The transition from 

 anteroventral (C) to posteroventral (D) margin is 

 usually an arc. I'^ach margin was measured to the 

 midpoint of the arc. 



(Quadrate (See fig. 81. Length (G) was measured 

 from the most ventral part of the articular condyle 

 to the tip of the spine. Width of horizontal edge (F) 

 was measur(>d along the liorizontal dor.sal edge to the 

 point where it forms a slight depression before the 

 si)ine. Total widtli (E) was measured from the 



anterior jioint of the horizontal dorsal edge along a 

 ])roj('ctioii of the line of this eilge to the posterior 

 margin of the spine. 



SOth vertebra. Length was measured along the 

 axis of the centrum from the outer anterior rim to 

 the outer posterior rim. Width was measured ver- 

 tically on the lateral sui'face at the narrowest part of 

 the centrum and does not include the neural or 

 haemal arches. 



Bone terminology is that of de Sylva (1955), 

 which has been used for tuna osteologj' by most other 

 recent workers. A number of names differ from 

 those accepted by many fish osteologists. 



PART 1. COMPARATIVE ANATOMY 



The morphological characters useful for distin- 

 guishing the species of Thinmus fall into seven 

 groups: osteology, viscera, vascular system, olfactor.v 

 organ, meristic characters, morphometric characters, 

 and coloration. These will be discussed in this 

 order in this section of the paper. The first three 

 groups, osteology, viscera, and vascular system, in- 

 clude the most important characters. 



OSTEOLOGY 



Osteological characters are very important in dis- 

 tinguisliing species of Thuiinufi. They have an ad- 

 vantage over characters in the soft anatomy in that 

 the bones can be saved so futui'e workers can reex- 

 amine the material on which a study is based. We 

 have used a large amount of the material Godsil and 

 his co-workers reported on. Four groups of osteo- 

 logical characters will be considered: neurocranium, 

 liranchiocranium, pectoral and pelvic girdles, and 

 verteliral column. The most us(>ful characters are 

 in the skull and the vertebral column. Each of the 

 four groups will be discussed separately, giving a 

 general osteological description followed by an enu- 

 meration of the specific charactei's. 



Neurocranium 



General characteristics. — Details of the general 

 neurocranial structure of tunas have been illustrated 

 by Masterman (1894), Kishinouye (1923), Frade 

 (1932), Gregory (1933), Godsil and Byers (1944), 

 de Sylva (1955), and Nakamura (1905). The ac- 

 companying labeled figures of an albacore (Thunnus 

 alalunga) skull show the bones of the neurocranium 

 (figs. 1-3). Photographs of the skulls of six of the 



seven species of Tlniiimif: are jjresented in appendix 

 figs. 1-3. 



The skull of a tuna, compared with that of most 

 typical percoid fishes, is short and wide. In dorsal 

 view (fig. 2), at its anterior end, the dermethmoid 

 (= ethmoid) is wide and its anterior margin only 

 slightly curved. The interorbital region is broad, 

 and the otic region broader still. A i)rominent 

 dor.solateral crest is formed on each side by the 

 frontal, parietal, and epiotic bones; each of the 

 epiotics bears a short, posteriorly directed process. 

 The lateral edges of the frontal and pterotic, making 

 up tlie sides of the roof of the neurocranium, form a 

 more j^rominent and rather fiat sharp crest on each 

 side which extends posteriorly as a long i)late-likc 

 pterotic spine. The supraoccipital crest is high and 

 extends posteriorly o\'er the first few vertebrae. 



\'eiitrally, the dentigerous vomer (= prevomer) 

 is flanked l)y broad processes of the parethmoids 

 (= lateral ethmoids). Most of the ba.se of the skull 

 is formed by the parasphenoid, which is flat or 

 slightly concave in its anterior two-fifths, bears a 

 medial, ventrally directed crest in the next two- 

 fifths, and posteriorly is first convex, then deeply 

 conca\e, with dorsomesially curved lateral flanges 

 that enclo.se a parasphenoid chamber. Lateral wings 

 jjioject dor.sad from near the end of the ventral crest 

 to form part of the posterior myodome. 



In lateral view, thealisphenoids (= pterosphenoids) 

 form a partial interorbital septum extending vontrad 

 from the roof of the skull. In extreme cases (large 

 Tluinniis thijnnus) this septum may be fused with 

 the j)arasphenoid, to form a bony septum partially 

 separating the orl)its. In the posterior part of the 



08 



U.S. FISH AND WILDLIFIO SEHVICE 



