Nos. IAND2.] AXATOMV OF SCOMBER SCOMBER. y^ 



nal to the dermal part of the squamosal, but external to the small 

 V-shaped process that is beginning to grow downward from the 

 latter bone along the outer surface of the cartilage of the skull. 

 In both fishes the extrascapulars lie external to the muscles. Since 

 the hind ends of the frontals, in both fishes, lodge portions of the 

 lateral canals, they must be, in both, of similar origin, and this is 

 doubtless true also of the parietals. The conditions presented by 

 the two fishes thus seem most decidedly to indicate different lines 

 of descent from some fish in which the trunk muscles had not as 

 yet invaded the dorsal surface of the skull. In Scomber the mus- 

 cles have pushed upward and forward between the front end of 

 the extrascapular and the hind ends of the parietal and frontal; 

 in Amia they have pushed forward below the latter bones. In the 

 dilatator groove, the dilatator operculi, which also invades the 

 dorsal surface of the skull in Scomber, pushes forward wholly ex- 

 ternal to the frontal but internal to the dorsal end of the suborbital 

 part of the main infraorbital canal. 



The Postorbital Ossification (PST) is an irregular ossifi- 

 cation of the dorso-lateral corner of the postorbital process of the 

 skull. It has three exposed faces, an anterior, a lateral and a 

 dorsal one. The anterior face looks forward and downward, and 

 forms part of the hind wall of the orbit. The lateral face looks 

 downward and laterally, and lies nearly at right angles to the 

 anterior one. TJie lateral, and larger part of the dorsal face of 

 the bone forms the lateral part of the middle portion of the floor 

 of the dilatator groove, lying between the frontal in front and the 

 squamosal behind. The remaining, mesial portion of the dorsal 

 face of the bone is thin, forms part of the roof of the cranial cav- 

 ity, and is almost entirely covered by parts of the frontal, parietal 

 and squamosal. That small portion of it that is not so covered 

 lies in the floor of the temporal groove, where it forms the lateral 

 part of the ledge that separates the antero-superior portion of the 

 groove from its posterior, depressed portion. A large anterior 

 process of the squamosal lies between this little exposed surface 

 of the bone and that part of the bone that is exposed in the bottom 

 of the dilatator groove. 



A broad, shallow and tapering groove runs mesially and for- 

 ward across the dorsal face of the ossification immediately lateral 



