FISHERY BULLETIN: VOL. 73, NO. 3 



SPHENOTIC 



PTEROTIC 



PARIETAL 



FRONTAL 



ETHMOID 



INTERCALAR 



EXOCCIPITAL 



VOMER 



FIRST VERTEBRA 



EPIOTIC 



LATERAL ETHMOID 



PINEAL FORAMEN 



SUPRAOCCIPITAL 



Figure lO.-Dorsal view of skull of Gymnosarda unicolor, Truk Islands, 6% mm FL. 



supraoccipital crest is high and usually does not 

 extend posteriorly past the tip of the first neural 

 spine. 



A prominent pineal foramen (Figures 10-14) is 

 present anterior to the supraoccipital crest 

 between the median edges of the frontal bones. 

 Among the bonitos, it is most prominent in 

 Allothunnus. Other scombrids either have a 

 reduction in the thickness of the bone in this area 

 {Scomber, Rastrelliger, and Scomberomorus) or a 

 similar pineal foramen (Grammatorcynus, 

 Acanthocybium, Auxis, Euthynnus, Katsuwonus, 

 and Thunnus). In life, the pineal foramen is filled 

 with a translucent cartilaginous lens and is 

 covered by a layer of skin usually having an un- 

 pigmented window dorsal to the cartilage. Rivas 

 (1953) hypothesized that, by permitting light to 

 reach the pineal body, the pineal apparatus in 

 Thunnu.s could be instrumental in controlling 

 phototactic movements involved in migration. 

 Holmgren (1958) concluded that while there are 



morphological indications of light sensitivity in 

 the pineal area of T. thynnus, there is not enough 

 physiological evidence to be sure of possible hor- 

 monal control by this area. 



Ventrally (Figures 15-19), the prelateral eth- 

 moid region of bonitos is shorter and broader than 

 in other scombrids. The ventral surface of the skull 

 is formed by the vomer, lateral ethmoids, and 

 parasphenoid and is broader than that of more 

 advanced scombrids. The anterior three-fifths of 

 the parasphenoid is almost flat, there is a medial, 

 ventral keel in the next fifth, and posteriorly it is 

 thin and smoothly curved into lateral flanges 

 which enclose a parasphenoidal chamber 

 (Kishinouye 1923). The lateral wings of the 

 parasphenoid project dorsolaterally from the 

 posterior half of the ventral keel to form the 

 anteroventral part of the posterior myodome. The 

 ventral surface of the brain case is formed by the 

 frontal, pterosphenoid, sphenotic, prootic, basioc- 

 cipital, exoccipital, intercalar, and pterotic bones. 



536 



