BASIOCCIPITAL 



OPISTHOTIC 

 PROOTIC 



FRONTAL 



DERMETHMOID 



PARETHMOID 



SPHENOTIC 

 ALISPHENOID 



f ,/■ . 



VOMER 



PARASPHENOID 



PTEROTIC 



EXOCCIPITAL 



PARASPHENOID 



Figure 3. — Skull of Thunnus alalunga. Ventral view. 



have been found useful in distinguishing the species 

 of Thunnus: the ahsphenoids, posterior parasphenoid 

 margin, supraoccipital crest, and ventral parasphe- 

 noid shaft. They are characteristic of specimens 

 from all oceans. 



Alisphenoids (fig. 4). The alisphenoids meet in 

 the median line and extend ventrad into the orbit. 

 They approach the parasphenoid more closely in 

 T. ihijnnus and T. maccoyii than in the other tunas. 

 The greatest height of the anterior part of the orbit, 

 B, measured from dorsal parasphenoid to upper 



median part of parethmoid, was divided by the least 

 distance between alisphenoid and para.sphenoid, A. 

 In 46 skulls of T. thynnus, 16 have the alisphenoids 

 fused to the parasphenoids; in the remaining 30, 

 A goes into B 2-15 times; with a mean (I-) of 4.8, 

 only in 6 specimens is the ratio less than 2.5. No 

 fusion was observed in T. maccoyii; in 17 skulls the 

 ratio was 2.0-10.3, x 4.8. By contra.st, in all other 

 species, A goes into B 1-3 times. Among 122 skulls 

 of the other species, only 3 T. albacares and 2 large 

 T. tonggol have a ratio of 2.5 or greater. Mean 



ANATOMY AND SYSTEMATICS OF TUNAS 



71 



