74 



ALUS. [Vol. XVIII. 



space has a free antero-mesial edge, but is elsewhere bounded by 

 the postorbital ossification, the squamosal, the exoccipitale, and 

 the supraoccipital. From it, as already stated, narrow fines of 

 cartilage run outward between the adjoining edges of the above 

 named bones. 



The hind edge of the frontal is irregular and jagged in out- 

 line, and the dorsal surface of the posterior portion of the bone is 

 deeply grooved to form the anterior portions of the three deep 

 grooves that here occupy almost the entire dorsal surface of the 

 skull. The ventral surface of the frontal gives little or no indica- 

 tion of these grooves. The hind edge of the bone overlaps the 

 anterior edge of the supraoccipital, and overlaps partly, and is 

 partly overlapped by the anterior edge of the parietal. The pos- 

 terior portion of its lateral edge overlaps the mesial edge of the 

 postorbital ossification. The two thin dorsal ridges of the bone 

 that separate the anterior ends of the three dorsal grooves, overlap 

 mesially corresponding ridges on the parietal and squamosal. 

 The parietal is only slightly so overlapped, the squamosal is over- 

 lapped to a considerable extent. On the summit of the fronto- 

 squamosal ridge, between the adjoining and overlapping edges of 

 the frontal and squamosal and extending forward onto the frontal 

 alone, there is a relatively wide groove which lodges a part of the 

 main infraorbital lateral canal. 



The frontal is traversed, through a large part of its length, by 

 the supraorbital lateral canal, the hind end of that canal lying in 

 those parts of the bone that form the anterior end of the floor 

 of the supratemporal groove and the anterior end of the floor and 

 also of the lateral bounding wall of the temporal groove. The 

 canal thus lies internal to the anterior ends of those anterior exten- 

 sions of the trunk muscles that fill the grooves. In Ainia the cor- 

 responding part of the canal lies external to the morphological 

 plane of the dorsal surface of the trunk muscles, although these 

 muscles do not extend far enough forward to come into direct 

 relations with the canal. In Scomber those parts of the parietal, 

 squamosal, and exoccipitale that form the floor of these same 

 grooves all have the same relations to the muscles that the frontals 

 have, that is, they lie internal to them. In Amia, the anterior ex- 

 tension of the trunk muscles lies internal to the parietal and inter- 



