COLLETTE and CHAO: SYSTEMATICS AND MORPHOLOGY OF THE BONITOS (SARDINI) 



towards its posterior margin which attaches to the 

 frontal bone. Gymnosarda has the strongest nasal 

 bone, with serrations along its anteroventral 

 margin. The nasal in Allothunnus is more elon- 

 gate than in other bonitos and its constricted 

 midportion is more prominent. 



ORBITAL REGION.-The orbit is surrounded 

 by the posterior wall of the lateral ethmoid, the 

 ventral side of the frontal, the pterosphenoid, 

 sphenotic, prootic, suborbital, and lachrymal 

 .bones. The left and right orbits are partially 

 separated by the basisphenoid. The sclerotic bones 

 enclose the eyeballs. 



Godsil (1955) found a difference in the ratio of 

 orbit height to length between southeastern and 

 northeastern populations of Sarda chiliensis. The 

 ratio in his material varied from 5.83 to 6.83 in 5 

 southeastern specimens (606-659 mm FL) and from 



3.40 to 4.50 in 6 northeastern specimens (439-563 

 mm FL). Following Godsil's definitions, length 

 was measured along the shaft of the parasphenoid, 

 from its juncture with the lateral ethmoid to the 

 point where the lateral wing of the parasphenoid 

 begins. Orbit height is the least distance between 

 the anterior ventral projection of the 

 pterosphenoid and the shaft of the parasphenoid. 

 The range of variation in this ratio (Table 4) in- 

 dicates that this is not a useful diagnostic 

 character within the genus Sarda, although the 

 means for the populations of S. chiliensis show 

 the same trend that Godsil found. Orcynopsis and 

 Cyhix>sarda have the highest ratios, which indicate 

 the most elongate and narrow orbits and the most 

 ventrally projecting pterosphenoids. Allothunnus 

 and Gymnosarda have the lowest ratios due to 

 their slightly higher arched orbits and the smooth 

 ventral margins of the pterosphenoids. 



LATERAL ETHMOID 



ETHMOID 



BASISPHENOID 

 PTEROSPHENOID 

 FRONTAL 



PTEROTIC 



NTERCALAR 



BASIOCCIPITAL 



FIRST VERTEBRA 



VOMER 



PARASPHENOID 



EXOCCIPITAL 



SPHENOTIC 



Figure 15.— Ventral view of skull of Cybiosarda elegans, Western Australia, 422 mm FL. 



541 



