52 ALUS. [Vol. XVIII. 



the anterior edge of the trunk muscles. In Alestcs the posterior 

 limits of this depressed region correspond exactly to those of the 

 groove in Scomber, but in most of the fishes described the depres- 

 sion includes the entire dorsal surface to the exoccipitale, its 

 posterior limits thus corresponding, approximately, to those of 

 the temporal and supratemporal grooves united of Scomber. The 

 temporal groove of this latter fish may, thus, be simply a part of 

 the superficial depression shown by Sagemehl in the fishes de- 

 scribed by him, and not at all the homologue of his temporal 

 groove, which groove is covered dorsally by parts of the adjoin- 

 ing bones of the skull. This marked disagreement in the rela- 

 tions of the temporal groove of Scomber to the bones that 

 bound it has already been referred to in an earlier work (No. 

 6,), and it was there stated that it seemed to indicate that Scom- 

 ber and Amia represented two different lines of descent from 

 some fish in which the tnmk muscles had not, as yet, invaded the 

 temporal regions of the skull. That the dorsal, bony covering 

 of the groove has simply disappeared in Scomber, thus leaving the 

 muscles exposed, seems improbable, for parts of the bones that are 

 covered by the muscles, in Scomber, are traversed by the lateral 

 sensory canals. 



The Lateral Surface of the brain case is, in its general outer 

 level, inclined at about 45° to the mid- vertical plane of the head. 

 The entire posterior portion of this surface is, however, occupied 

 by a deep depression, the center of which lies on a level with the 

 dorsal surface of the vertebral column and slightly in front of the 

 front end of that column. This depression gives to the hind end 

 of the skull a pinched appearance, and to the dorsal part of its 

 lateral surface, at its hind end, an inclination of something like 

 70° to the vertical plane. The depressed region is, apparently, 

 in no way comparable to the subtemporal hole of cyprinoids 

 (No. 66, p. 554), and apparently owes its existence simply to the 

 entire absence of a bulla acustica. The saccular recess of the 

 cranial cavity lies internal to the deepest part of the depression. 



The Posterior Surface of the skull is relatively small, because 

 of the four large dorsal grooves that cut into its dorsal portion, and 

 it presents no special characteristics. Its mesial third, on each side, 

 forms a slightly depressed region which is continuous, at its upper 

 end, with the hind end of the supratemporal groove. 



