These tendencies exist, but frequent exceptions ren- 

 der the character uncertain. 



A depression in the dorsal profile just anterior to 

 the supraoccipital, reported to be present in all 

 species but most pronounced in T. thtjnnus, is related 

 to the pineal foramen. We find this variable in all 

 species and distinctive in none. 



In the contour of the posterior margin of the 

 sphenotic as seen in ventral view, we can detect no 

 difference among the species. A concave curvature 

 of the margin, held by Godsil and Bj'ers to be 

 characteristic of T. alalunga and T. thynnus, is not 

 only slight but may be present or absent in all species. 



Branchiocranium 



General description. — The branchiocranium in- 

 cludes the branchial bones, opercular apparatus, 

 jaws, and associated bones. On each side the den- 

 tigerous premaxilla forms the upper jaw, and the 

 maxilla is located dorsomesial to it. A small sup- 

 lamaxilla is attached to the posterior end of each 

 maxilla. The lower jaw includes the dentary, which 

 bears teeth; the articular, forming the rear end of 

 the jaw and articulating with the condylar region of 

 the quadrate; and a small angular at the poster- 

 oventral corner of the articular. The suspensorium 

 begins with the hyomandibula, which articulates at 

 its upper end with the otic region of the neurocranium 

 and with the opercle. The ventral limb of the 

 hyomandibula articulates with the metapterygoid, 

 and the ventral portion of the latter in turn articu- 

 lates with the symplectic and quadrate. To the 

 anteroventral part of the metapterygoid are joined 

 the ba.'ial portions of the endopterygoid and ectop- 

 terygoid. At their anterior ends, these are joined 

 to the short, dentigerous palatine, which articulates 

 with the condyle of the anterior end of the maxilla. 



In addition to the hyomandibula, the hyoid arch 

 is composed of two median and four paired bones. 

 A glossohyal supports the tongue, and a urohyal lies 

 below and between the two sides of the arch. The 

 paired bones include small basihyals, large cera- 

 tohyals that articulate with smaller epihyals by 

 jagged, toothlike sutures on the mesial side only, and 

 small interhyals posteriorly joining the operculum. 



In the branchial arches are three median basi- 

 branchials (a small cartilage posterior to the third 

 may represent a fourth ba.sibranchial, see Iwai and 

 Nakamura, 19r)4a) and on each side three hypo- 

 branchials, five ceratobranchials, four epibranchials, 

 and four pharyngobranchials. The posteriormost 



ceratobranchials and mesial three pairs of phar3'ngo- 

 branciiials bear villiform teeth. The anterior sur- 

 faces of the first four arches bear gill rakers, and, as 

 supports for the gill filaments, so-called gill bars are 

 found on the posterior surfaces (Iwai and Xakamura, 

 19G4a). 



Specific Characters. — Differences worthy of note 

 have been described for only six bones by Kishinouyc 

 (1923) or Godsil and Byers (1944). These are the 

 hyomandibula, metapterygoid, quadrate, subopercle, 

 interopcrcle, and ceratohyal. We concur in their 

 observations on the first four only. 



The anterior (sphenotic) articulating head of the 

 hyomandibula (fig. (!) is relatively longer and nar- 



FiornE 6. — Hyomriiidibula of (left) Thunnus thynnus, skull 

 length 130 mm., (right) T. alalnruja, skull length 150 mm. 

 Measurements of anterior articular head include A — least 

 width, H— length. 



rower in T. alalunga than in the other species. The 

 proportion of length to least width in our specimens 

 ranged as follows: T. alalunga {N = 3o) 1.7-2.7, 

 x='2.2; T. thynnus (iV=44) 1.3-2.1, x=1.7; T. mac- 

 coyii (iV=17) 1.6-2.3, x=1.9; T. obesus (.V=36) 

 1.3-1.9, x= 1 .5; T. atlanticus {N= 18) 1.3-2.2, x= 1.7; 

 T. albacares (.V=58) 1.2-1.9, x=1.6; T. tonggol 

 (.V=4) 1.4-1.8, x= 1.6. These proportions are close 

 to those given by Godsil and Byers (1944: 86), who 

 reported 1.7-3.0 for Pacific T. alalunga and 1.2-1.5 

 for Pacific T. albacares, and stated that Pacific 

 T. thynnus and T. obesus are similar to T. albacares. 



74 



U.S. FISH .\N'D WILDLIFE SERVICE 



