FISHERY BULLETIN: VOL. 73, NO. 3 



SUPRAOCCIPITAL PTEROTIC 



ETHM 



NTERCALAR 



OCCIPITAL 



FIRST VERTEBRA 



VOMER 



FHMO 



PINEAL FORAMEN 



PARIETAL EPIOTIC 



Figure 12.-Dorsal view of skull of Sarda sarda, eastern United States, 388 mm FL. X's indicate the points of attachment of 



intermuscular bones. 



(Mago Leccia 1958), Katsuwonus, and Thunnus. In 

 Cybiosarda and Orcynopsis, these anterofrontal 

 crests are merely visible laterally as in Auxis and 

 Euthynnus. No anterofrontal crest is present in 

 Gymnosarda and Allothunnus (Figures 22, 24) and 

 it is also absent in Grammatorcynus and 

 Acanthocybium. 



The first vertebral centrum has modified neural 

 prezygapophyses (Figure 25) that firmly attach 

 the first vertebra to the modified exoccipital and 

 basioccipital region (Figure 26) of the skull. The 

 anterior circular margin of the first centrum is 

 smaller than its posterior margin and is in a for- 

 ward oblique position to meet the posterior margin 

 of the modified basioccipital which is in a back- 

 ward oblique position. The strong first neural 

 spine, with its tubular base, sits on the junction of 

 the first centrum and exoccipital and it is not fused 

 to the first centrum. No bony shelf is present in the 

 neural canal (Figure 25a) to divide it into dorsal 

 and ventral portions as is the case in Auxis, 

 Euthynnus, and Katsuwonus. An intermuscular 

 bone is attached to the center of each exoccipital in 

 the bonitos (also see section on ribs and intermus- 



cular bones) except in species of Sarda (Figure 26) 

 which have an additional pair of intermuscular 

 bones attached to the exoccipital just anterior to 

 the first neural prezygapophyses and Gymnosarda 

 which lacks cephalic intermuscular bones. 



The prootic pits (Godsil 1954) of the more ad- 

 vanced scombrids are incipient in bonitos except in 

 Allothunnus, which has a larger pouchlike con- 

 cavity on each side of the ventral surface of the 

 cranium. Part of the roof, floor, and sides of the 

 prootic pit are formed by the prootic and pterotic 

 bones. The branchial musculature originates in 

 these pits in Thunnus (Gibbs and CoUette 1967). 

 The posterior end of the orbital region extends 

 into a deep median depression, the posterior 

 myodome, which is surrounded by the prootic 

 anterodorsally, the parasphenoid ventrally, and 

 the basioccipital postdorsally and postlaterally, 

 and then opens at the back of the skull between the 

 posterior flanges of the parasphenoid, or com- 

 municates with a posterior or parasphenoidal 

 chamber (Kishinouye 1923). The rectus muscles of 

 the eyes attach in the posterior myodome (Gibbs 

 and CoUette 1967). 



538 



