Nos. IAND2.] ANATOMY OF SCOMBER SCOMBER. 67 



ing only a narrow lateral edge, is covered by a thin layer of the 

 cartilage of the antorbital process of the chondrocranium. 



From near the mesial end of the thin, dorso-posterior edge of 

 the preorbital ossification a strong rib, or brace, runs downward 

 and slightly inward to the ventral edge of the bone. It is flat, 

 projects directly backward, or backward and inward, into the 

 orbit, and may be called the orbital rib of the bone. It divides 

 the posterior, or orbital face of the bone into two distinct regions, 

 an external and an internal one. From the base of the lateral 

 surface of the rib, and at about the middle of its length, a strong 

 ligament arises, and, running downward, is inserted on the exter- 

 nal surface of the palato-quadrate immediately posterior to the 

 surface by which that arch here articulates with the skull. It 

 holds the arch up against the ventral, articular edge of the postero- 

 lateral process of the preorbital ossification, and allows a swinging, 

 latero-mesial movement along that edge. The space, internal to 

 the orbital rib of the bone forms the extreme anterior end of the 

 orbit, and has a concave surface. In it, slightly dorso-mesial to 

 its middle point, is the posterior opening of the canal by wHich 

 the olfactory nerve traverses the antorbital process, the canal lying 

 entirely in the preorbital ossification. A short, straight ridge runs 

 downward and inward tangential to the ventro-lateral edge of the 

 opening ; and ventro-lateral to this ridge, between it and the orbital 

 rib, there is a deep recess. The anterior opening of the olfactory 

 canal lies immediately internal to the angle formed between the 

 dorso-anterior and ventro-anterior arms of the X of the preorbital 

 ossification. That part of the bone that lies between these two 

 arms forms the posterior part of the floor and the corresponding 

 part of the mesial wall of the nasal pit. 



The preorbital ossification is separated, anteriorly, from the eth- 

 moid bone by a narrow line of cartilage ; and the mesial edge of 

 its orbital surface is separated by a similar line from the pre- 

 orbital ossification of the opposite side of the head. The outline 

 of the bone, where it adjoins the cartilage, is circular, the center 

 of the circle being approximately the centre of the four arms of 

 the X of the bone. The ventro-lateral edge of the bone may be 

 slightly overlapped by the lateral edge of the parasphenoid. 



