KatsuwonidSp When we compare therrij a course of development leading 

 from the Soombridae through "chs Cybxidae to the Thi-nnidae is 

 clear '-J apparent o 



In the Soombridae the skeleton is generalij' frail and weakly 

 articulated, while in the Thunnidae the skeleton is strong and 

 the articulations are solido 



The skull is roughly pyramidal in shape and is pointed 

 nnterioriy,. There are five conspicuous projections at the rear. 

 Among these the occipital crest in the center is particularly cons- 

 picuous » Lateral to it are a pair of temporal crestSo Lateral 

 to these are the pt erotic crests j and there are deep grooves 

 between these projectionso Looking at the form of the skull as 

 a whole, its length is markedly less than in the Soombridae and 

 Cybiidae and it is only a little longer than it is wideo The 

 breadth of the skull is far greater than its deptho 



There is one ethmoid bone located anteriorly and in the 

 middleo Above it is the frontal bone, and to the side and rear 

 are the prefrontalso Below are the vomer and the parasphenoido 

 On the left and right there are a pair oi" hornshaped projections, 

 the ventral surfaces of which articulate v/ith the maxiilaryo The 

 prefrontal forms the anterior wall of the eye-socket ^ and in the 

 center of it there is a foramen for the passage of the olfactory 

 nerveo There is one vomer at the farthest anterior edge of the 

 skull o Its anterior part is thickened and on the ventral surface 

 of the center of the thick portion there are villiform teethe 



There is a pair of large frontal boneso They are fused 

 together on the center line and form a bridge above the eye-=socketSo 

 Their anterior portions are flat^ thin, and narrow, but the posterior 

 portions are broad and are bent somewhat ventral ly^ The anterior 

 portions cover the dorsal surface of the prefrontals conipletely 

 and extend onto the posterior part of the ethmoido The posterior 

 part articulates wit?i or is fused to the supraoccipitali, the 

 parietals, the sphenotic, the pterotic, and the alisphenoids,. 

 The posterior edges of the frontals are not fused together over 

 the alisphenoids and form a conspicuous forameno 



The alisphenoids are paired bones which fonii the anterior 

 part of the floor of the brain casOo The left and right bones 

 are not perfectly fused togetht,-, but form a large openingo The 

 anterior parts of these bones connect with the frontals „ the 

 posterior parts with the prootic and basisphenoticj, and the 

 lateral portions with the sphenotico On their dorsal sides they 

 are in contact with the supraoccipitalo 



The parasphenoid is a long narrow bone which forms the 

 greater part of the ventral side of the skullc Except for its 

 anterior end and posterior portion it is almost free of contact 

 with other boneSo Its anterior end connects with the vomer, the 

 frontals, and the basisphenotico Near its posterior end two 



4 



