Nos. IAND2.] AXATOMV OF SCOMBER SCOMBER. 71 



Immediately posterior to the hind end of the antorbital carti- 

 lage, the frontal forms, for a short distance, the entire roof of the 

 orbit of its side. Near the mesial edge of the ventral surface of 

 this part of the bone, at about the middle of the entire length of 

 the bone, and at about the middle of the orbit, a strong double ridge 

 begins (Fig. 20). Running at first backward and but slightly later- 

 ally, this ridge soon begins to gradually curve laterally, and contin- 

 uing in a more and more lateral direction reaches the lateral edge 

 of the bone at the bottom of the curved or angular piece cut out of 

 it to expose the postorbital ossification. The anterior surface of 

 this ridge slopes downward and backward in a gentle curve, which 

 is continuous, ventral to the ridge, with the orbital surfaces of the 

 alisphenoid and the postorbital ossification. At about the middle of 

 the length of the ridge, on its posterior aspect, a short branch 

 ridge runs mesially, or mesially and slightly backward, to the 

 antero-lateral corner of the median, post-epiphysial interspace of 

 cartilage which has already been referred to and is described below. 

 The entire ridge has thus somewhat the shape of a letter Y. The 

 shank of this Y, and its posterior arm, are both double throughout 

 their entire length, each being formed by two parallel, laminar 

 processes which are directed backward and downward and enclose 

 between them a narrow open space. The anterior arm of the Y is 

 similarly double at its base, but not at its anterior end, the two 

 laminar processes there uniting to a single ridge, thus leaving 

 enclosed between their posterior portions a small wedge-shaped 

 open space. The two laminar processes that form the posterior 

 arm of the Y spread, when they reach the post-epiphysial car- 

 tilage, and lying nearly at right angles to each other, diminish 

 gradually in height. The whole structure may otherwise be said 

 to be formed by a combination of three thin laminar processes, 

 one of which is large and slightly curved, and the other two some- 

 what \'-shaped. The slightly curved process forms the anterior 

 surface of the ridge and Y. The two \'-shaped ones lie, one 

 between the two arms of the Y, and the other between the pos- 

 terior arm and the shank of the Y. The space enclosed between 

 these three thin processes is thus Y-shaped with a relatively long 

 shank and very short arms. Into this Y the dorsal end of the 

 alisphenoid is received, that end of that bone being slightly ten- 



