142 WILLIAM F. ALLEN 



upper portion of the muscle, and both send off numerous branches 

 into the muscle. The dorsal ramus after lea\dng the myotomes 

 is represented as dividing into a branch for the dorsal fin region 

 and a branch designated as the ramus cutaneus superior, which 

 passes laterad and then ventrad along the inner surface of the 

 skin. At the ventral margin of the myotomes the ramus ven- 

 tralis sends off a branch to the obliquus and rectus muscles, and 

 a sensory bundle continues onward a short distance to separate 

 into a dorsal branch designated as the ramus cutaneus medius 

 and a ventral branch designated as the ramus cutaneus in- 

 ferior. Both rami follow the inner surface of the skin, the former 

 pursuing a general dorsal course and the latter after crossing 

 the mucous sac ends in the anal fin region. 



As a result of a study of the elements of a spinal nerve from a 

 Myxine series, taken from the region of the anus, I have been 

 able to confirm everything Cole figured in his transverse sec- 

 tion (fig. 1), excepting the union between the dorsal motor and 

 the dorsal sensory rami. In this region the motor ramus was 

 the more cephalic, and it was clear from the sections that it 

 terminated in the myotome before the sensory ramus reached 

 the same level. 



Realizing that some of the discrepancies in the descriptions 

 of the distribution of the motor and sensory components of the 

 spinal nerves might be due to a study of different regions and 

 to different genera, I began a careful study of the spinal nerves 

 at different levels of adult .Polistotrema (Bdellostoma) . A 

 reconstruction was made of the first two spinal nerves appear- 

 ing behind the auditory capsule. A second reconstruction was 

 made of a spinal nerve in the region of the caudal end of the M, 

 retractor mandibulae. Three reconstructions were made of a 

 number of spinal nerves in the tail region, one of which was from 

 a late embryo, and finally a dissection was made of several of 

 the spinal nerves in front of the caudal heart. Since consid- 

 erable variation is shown in the distribution of the motor and 

 sensory components in these reconstructions, a complete descrip- 

 tion will be given of each, beginning with the most cephalic. 



