292 ALUS. [Vol. XVIII. 



the posterior nerves. The first nerve then sends a branch to the 

 nervus pterygiaUs, to be described below, the remainder of the 

 nerve running downward along the inner surface of the trunk 

 muscles, immediately posterior to the clavicle, and entering the 

 muscles of the ventral fin. The second nerve, after sending its 

 first branch into the muscle of its segment, sends a long branch 

 downward and backward along the inner surface of its segment. 

 It then sends a branch forward to the ventral branch of the first 

 spinal nerve, the branch joining the latter nerve distal to the 

 point where the communicating branch is sent to the nervus 

 pterygialis. The remainder of the second nerve then continues 

 downward and forward along the inner surface of the trunk 

 muscles and enters the ventral fin. 



The two roots of each of the spinal nerves issue through 

 foramina in the vertebra that gives attachment to the inter- 

 muscular septum that forms the anterior boundary of the muscle 

 segment that the nerve innervates. The first spinal nerve 

 thus innervates the segment that lies between the first and second 

 vertebrae, that is the fifth muscle segment of the trunk. 



The Occipital Nerves are three in number, two of them hav- 

 ing dorsal and ventral roots, and one of them, the anterior one, 

 a ventral root only. These roots pierce the occipitale laterale by 

 one, two, or several openings, a separate foramen for each of the 

 five roots having been found in one fish. Immediately outside 

 the skull the several roots all enter a ganglionic formation that 

 is partly double, as if it had been formed by the fusion of two 

 ganglia. This ganglion lies immediately posterior to the third 

 intermuscular septum, and from it, in the one specimen in which 

 the dissection was successfully made, eight branches arose. 

 Three of these branches were dorsal branches, two of them hori- 

 zontal branches, and three of them ventral branches. The most 

 posterior of the three dorsal branches took the same general 

 course in relation to the fourth muscle segment that the dorsal 

 branches of the spinal nerves do to the segments to which they are 

 related. The next anterior one ran almost directly upward, 

 pierced the third intermuscular septum and then turned upward 

 and backward in the third muscle segment. The first, or most 

 anterior one pierced at once the third intermuscular septum, trav- 



