SUPRAOCCIPITAl 

 CREST 



EPIOTIC 



PARIETAL 



FRONTAL 



DERMETHMOID 



PTEROTIC 



VOMER 



PARETHMOID 



PARASPHENOIO 



ALISPHENOID 



OPISTHOTIC 

 EXOCCIPITAL 



FIRST CENTRUM 

 PROOTIC 



BASIOCCIPITAL 



SPHENOTIC 



BASISPHENOID 



Figure 1. — Skull of Thunnus alalunga. Lateral view. 



orbital region is a median, vertically oriented basi- 

 sphenoid, which usually has an anteriorly directed 

 process near its dorsal end. The posterior base of 

 the cranium is formed by the end of the parasphenoid 

 (ventral profile) and the lateral flanges of the basi- 

 occipital (posterior profile). The first vertebra artic- 

 ulates firmly, partly by a jagged suture, with the 

 occipital region and forms an integral part of the 

 skull. One end of Baudelot's ligament attaches to 

 the basioccipital, the other end to the supracleithrum. 

 The prootic pits (Godsil, 1954) are large pouchlike 

 concavities on each side of the ventral surface of the 

 cranium, opening posteriorly and separating the 

 pterotic bones from the brain case. Part of the 

 roof, floor, and sides of each prootic pit is formed by 

 the prootic bone, and the anterior wall by the 

 sphenotic. The pits function as areas of attachment 

 for the branchial musculature. These pits are char- 

 acteristic of the most advanced scombrids — Thunnus, 

 Euthynnus, Katsuwonus, Auxis, and Allothunnus — 



and are incipient in Sarda (Starks, 1910; Godsil, 

 1954). 



The posterior myodome is a deep median depres- 

 sion opening anteriad at the posterior end of the 

 orbital region. Its anterolateral walls and roof are 

 formed by the prootics, its floor and ventrolateral 

 walls by the parasphenoid, and its posterior concave 

 wall by the basioccipital. The posterior myodome 

 functions as a place of attachment for the rectus 

 muscles of the eyes. The narrow basisphenoid lies 

 just anterior to the anterior opening of the posterior 

 myodome. There is a posterior or parasphenoidal 

 chamber (Kishinouye, 1923), which communicates 

 with the posterior myodome and is formed by the 

 upcurved walls of the posteriormost end of the 

 parasphenoid. 



A large triangular fronto-parietal foramen (lateral 

 parietal foramen of Masterman, 1894) is present on 

 each side of the dorsal surface of the skull, at the 

 junction of the frontal, parietal, and supraoccipital 



ANATOMY AND SYSTEMATICS OF TUNAS 



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