THE LATERAL CANAL SYSTEM. 353 



the system in Plagiostoiucs does not appear to have been changed to any 

 extent by deep sea conditions. 



Among the bony fishes the history of the sj'stem is somewhat dii'forent. 

 The arrangement of the canals diii'ers from that on the typical Plagiostome 

 to some extent, as is amply shown in the subjoined illustrations. The 

 separation of the canals of the right side from those of, the left is more 

 general and the presence of a spiracular canal on the operculum behind the 

 postorbital and usually connected with the oral is a common feature. This 

 opercular canal corresponds with the spiracular (sp) of Chlamydoselachus, 

 Plate LXX. In many if not most it is' apparently separate from the main 

 longitudinal canal at the side of the skull. Along the side of each mandible 

 there is a canal which is identified with the oral (o) of the typical Galei 

 (Antacea). An aural connection across the crown between the right side 

 and the left may have existed in numerous cases ; if so it has to a greater 

 or less degree become obsolete. A submental connection between the two 

 orals, as on Chaunax, Plate LXXIII. fig. 1, appears to be exceptional. 

 Accepting as typical the presence of both spiracular and postorbital, as 

 seen on Lamprogranimus, Plate LXXXI. fig. 1, and on the majoritj' of the 

 figures on Plates LXXII. to LXXXIV., anomalous developments are to be 

 noted on the more specialized forms, such as the changes brought about by 

 a shortening of the head behind the ej^es, as seen on the Scorpajnoid figured 

 on Plate LXXI. fig. 1, on which the spiracular and the postorbital canals 

 have fused (see Chaunax also, PI. LXXIII.), or on the Cottoid of the same 

 Plate, fig. 2, on which the spiracular joins the orbital, forming a plan inter- 

 mediate between that of the Scorpojnoid, fig. 1, and that of the Berycoid, 

 fig. 4, of the same plate. "Whether the aural branches are primarily to be 

 regarded as a loop, as on Lamprogranimus, Plate LXXXI. fig. I, leaving 

 the main canal opposite the parietal region and joining that canal again 

 forward of the junction of the postorbital is an open question. Possibly the 

 aui-al branches cross the head in some highly differentiated forms, like the 

 ScorpjBnoid and the Cottoid cited above, while on others, as Lampro- 

 grammus, Plate LXXXI. fig. 1, the loop represents simply another phase 

 of differentiation. Owing to the amount of maceration the specimens have 

 suffered it is difficult in many cases to trace the minute connecting thread 

 running from disk to disk in the canals ; for this reason in a number of the 

 figures of specimens on which it was nearly or quite invisible no attempt 

 has been made to place it ; its course can safely be supplied from the posi- 



