Nos. IAND2.] ANATOMY OF SCOMBER SCOMBER. 89 



In Alepoccphalus, Gegenbaur (No. 29) describes a bridge of 

 bone that separates the facial foramen into two parts. The tri- 

 geminal foramen is said to lie in front of the anterior one of 

 these so-formed facial foramina, near the anterior edge of the 

 petrosal, on what is described as the orbital surface of that bone. 

 This disposition of the three foramina seems to indicate that the 

 anterior of the two facial foramina described serves for the pas- 

 sage of the united ophthalmicus facialis and buccalis facialis; 

 the posterior opening serving for the passage of the truncus 

 hyoideo-mandibularis facialis. Such being the case, the space 

 enclosed beneath the bridge of bone in Alepocephaliis seems to be 

 the exact homologue of the trigemino-facial chamber of Scomber, 

 excepting that, in being smaller, the trigeminal foramen is not en- 

 closed within it. 



The internal surface of the petrosal of Scomber is concave and 

 irregular. Starting, approximately, at the base of the hind edge 

 of the horizontal process of the bone and running across the bone, 

 from behind, upward and forward, there is a ridge, more or less 

 developed in different specimens, which separates the surface into 

 two nearly equal regions. The several foramina that pierce the 

 bone all lie in front of, or antero-mesial to, this ridge ; the labyrinth 

 lies behind, or postero-lateral to it. It is, accordingly, a remnant 

 of the anterior wall of the labyrinth chamber, and is strictly 

 homologous with the more pronounced ridge that marks the posi- 

 tion of that wall in Amia (No. 4, Fig. 11, PI. XXI). Immedi- 

 ately lateral to the posterior third of this ridge there is a deep, 

 sharp groove (Fig. 63), which forms the anterior portion of a long 

 groove that lodges the sacculus. The extreme anterior end of the 

 groove runs forward a little beyond the anterior edge of its cere- 

 bral opening, and ends blindly in the body of the petrosal a little 

 below the middle point of the bone. Dorso-lateral to the anterior 

 end of the ridge there is a deep, oval depression which lodges the 

 ventral end of the anterior semicircular canal. This depression 

 is connected with the anterior end of the saccular groove by a 

 shallow groove or depression which lodges that part of the mem- 

 branous ear from which the external semicircular canal takes its 

 origin. Immediately dorsal to this last depression the thick dorsal 

 edge of the petrosal is deeply cut out, on its cerebral margin, a 



