VISCERAL ARCHES OF THE GNATHOSTOME FISHES. 381 
branches. These branches also at first lie on the extern 
surface of the posterior portion of the interhyoideus, and send 
branches to that muscle and to the adjacent portions of the 
continuous, dorso-ventral fibres of the constrictor super- 
ficialis ; these branches anastomosing more or less with each 
other. When the nerve and its branches reach the region 
where the primarily single muscle- sheet separates, along its 
lateral edges, into a superficial intermandibularis and a deeper 
interhyoideus portion, they, in four of the five specimens 
examined, all perforate the muscle-sheet, either at that line 
of separation or immediately anterior to it, and acquire a 
position between the two sheets. In this position the several 
branches either remain independent or unite to form one or 
two nerves, and, in one specimen which was examined simply 
for the muscles and not the nerves, they are shown, in my 
drawings, always lying between the two muscles, internal to 
the one and external to the other. In the other three of these 
four specimens, which were more carefully examined, the 
nerve or nerves ran forward for a certain variable distance 
between the two muscles, sending branches to them, and then 
perforated the intermandibularis, this time from within out- 
ward, and, reaching its external surface, there ran forward 
nearly to its anterior end. At I his point, all the branches 
again penetrated the muscle-sheet, which was here repre- 
sented by the intermandibularis alone, and did not again 
reappear on its external surface. In several instances the 
mesial branches of the nerves of opposite sides fused in 
the median line, anterior to the interhyoideus portion of the 
muscle, to form a single median nerve which then entered the 
musculus intermandibularis and was not farther traced. 
On one side of the fifth one of the five specimens a large 
branch of the nerve remained on the external surface of the 
muscle-sheet, the main nerve perforating the sheet and 
running forward in the manner above described. The large 
branch crossed onto the external surface of the musculus 
adductor mandibulae, sent a large branch to anastomose with 
the nervus mandibularis trigemini, and then itself turned 
VOL. 63, PART 3. NEW SERIES. 27 
