Nos. IAND2.] AX ATOMY OF SCOMBER SCOMBER. 85 



the latter portion being- only one third or one half as long-, in an 

 antero-posterior direction, as the former. The anterior portion 

 is flat, projects mesially and slightly forward, and lies in a plane 

 inclined upward at about 45° to the plane of the posterior portion. 

 Its hind edge forms the lateral portion of the anterior edge of the 

 pituitary opening, and its mesial edge articulates with the lateral 

 edge of the basisphenoid. The mesial edge is deeply cut out, 

 usually at about the middle of its length, by the foramen for the 

 nervus oculomotorius. This portion of the process forms, as does 

 the posterior portion, a part of the floor of the cranial cavity and a 

 corresponding part of the roof of the eye-muscle canal. Its dorso- 

 anterior edge lies in a nearly horizontal position, is directed 

 mesially and but slightly forward, and forms that part of the 

 petrosal that articulates with the alisphenoid. The petrosal thus 

 articulates with the alisphenoid by a part of its mesial, horizontal 

 process, and not by a part of the body of the bone. The ventral, 

 or ventro-anterior surface of the anterior process forms a direct 

 continuation of the external surface of the alisphenoid, the two 

 bones being separated by a nearly horizontal line, which may be 

 considered as marking the indefinite limit between the hind wall 

 of the orbit and the roof of the eye-muscle canal. The anterior 

 process can be called, for convenience, the basisphenoid process 

 of the bone, the term horizontal process, or wing, being retained 

 for the posterior portion alone. 



Ventral to the horizontal process of the petrosal, the body of the 

 bone is as thin as the process itself, and has as much the appear- 

 ance of being a process of the bone, as the so-called process. The 

 ventral half of the Entire bone, in fact, presents decidedly the ap- 

 pearance of a thick bone, the ventral edge of which has been 

 deeply hollowed out by the strongly developed eye-muscle canal. 

 In Salmo, the ventral part of the body of the petrosal is said by 

 Parker (No. 50, p, 102) to be preformed as a cartilaginous lamella, 

 which grows downward, on each side, from the investing mass, 

 "thus forming a covered archway," The horizontal process of 

 the petrosal is thus preformed, in Salmo, before the ventral part 

 of the body of the bone. In Amia, on the contrary, the body of 

 the bone is preformed before the first appearance of its horizontal 

 process (Xo. 4, p. 505). 



