ALB 



ATL 



J 



TON 



FlouRK 4. — Skulls of Thunn Right lateral view of orbital ronion of neurocrimium showinf; conformation of basi- 



sphenoid (stippled) and ai.. , -J. Arrangement within each species in order of increasing skull lengths from left to 



right: ALA— T. alabmga, 131, 138, 150, 107 mm. ALB— T. albacimx, 103, 110, 122, 179 mm. XV\.— T. atUintiais, 51, 

 SO, 97, 102 mm. OBE— 7". ohesus: upper-Atlantic, 170, ISl, 185, 207 mm.; lower-Pacific, 07, 142, 147, ca. 240 mm. 

 THY— T. thynnus, 76, 139, 231, ca. 320 mm. TON— T. tonggol, 57, 61, 122, 128 mm. 



values arc: T. tonggol (.V=8) 2.0; T. alhacarcs 

 (A^=43) 1.8; T. ohesus {N=32) 1.6; T. alnhmgn 

 (.V=27) l.G; T. ailanlicus (iV=14) 1.2. 



Posterior parasphenoid margin (fig. 5). Tlie jjor- 

 tion of the parasphenoid forming the walls of the 

 parasphenoidal chamber is partialh- covered laterally 



by tlic basioccipital. Together tlie margins of these 

 bones extend vcntrad from the first vertebra, either 

 vertically or slanting forward or backward ; anteriorly 

 the parasphenoid alone forms the margin. With 

 due consideration for growth changes and individual 

 variation, the profile formed at this part of the skull 



72 



U.S. FISH AND WILDLIFE SKUVICE 



