FISHERY BULLETIN: VOL. 73, NO. 3 



basisphenoid curves posteriorly as it approaches 

 the parasphenoid and is not attached to the 

 parasphenoid in Allothunnus (Figure 25). This 

 may explain why the basisphenoid was lost in 

 three of the four skulls that we prepared. The 

 angle of the anterior basisphenotic process was 

 used as a character in Thunnus by de Sylva 

 (1955:32-35), but Gibbs and Collette (1967:73) 

 found it too variable to be useful. In bonitos, the 

 axes of the anterior and the posterior processes of 

 the basisphenoid are straight, with the anterior 

 process slightly directed downward in some 

 specimens. Allothunnus has no basisphenotic 

 process and the anterior process in Orcyncypsis and 

 Cybiosarda is not as prominent as in the other 

 bonitos. The angle between the axis of the process 

 and the vertical axis of the basisphenoid is similar 

 in all bonitos except for Gymnosarda (Figure 19). 

 In Gymnosarda, a ridge is present along the axis 

 of the process and the posterior process is more 

 ventrally directed. 

 Among other scombrids, Thunnus and 



Scomberomorus lack a prominent posterior process 

 on the basisphenoid (Gibbs and Collette 1967; 

 Mago Leccia 1958, pi. 3), Allothunnus and Ras- 

 trelliger have no process on the basisphenoid 

 (Nakamura and Mori 1966; Gnanamuttu 1971, fig. 

 1), Acanthocybium, Auxis, Euthynnus, and Kat- 

 suwonus have both the anterior and the posterior 

 process. If the shape of the basisphenoid bone is 

 used as a diagnostic character in scombrid clas- 

 sification (Kishinouye 1923; Godsil 1954; de Sylva 

 1955; Gibbs and Collette 1967), its high in- 

 traspecific variability must be taken into account. 



Pterosphenoid.-The pterosphenoid (alis- 

 phenoid) articulates with the edges of the frontal, 

 sphenotic, prootic, and basisphenoid on the ventral 

 surface of the brain case. Anteriorly, the 

 pterosphenoids are connected to the lateral eth- 

 moids directly or indirectly by a cartilaginous 

 membrane. The paired pterosphenoid bones meet 

 each other anteriorly along the median ventral 

 line forming a pterosphenotic window that differs 



PROOTIC 



SPHENOTIC 



PTEROTIC 



FRONTAL 



LATERAL ETHMOID 



ETHMOID 



INTERCALAR 



VOMER 



FIRST VERTEBRA 



EXOCCIPITAL 



PARASPHENOID 



PTEROSPHENOID 



BASISPHENOID 

 Figure 18.- Ventral view of skull of Sarda sarda, eastern United States, 388 mm FL. 



544 



