No. 2.] MORPHOLOGY OF THE MYXINOIDEI. 215 



The fin-rays are imperfectly segmented, and are surrounded by 

 a sheath of connective tissue, which is also constricted in 

 places, but not always in agreement with the cartilage seg- 

 ments. The bodies of the fin-rays are conical, their bases 

 being the proximal ends. They are often bifurcated at their 

 distal ends, sometimes twice, so that each ray may have three 

 or four terminal twigs. 



In the anterior portions of the fins one can recognize a gen- 

 eral segmental arrangement of the fin-rays, though the agree- 

 ment between myotomes and fin-rays as well as gills, slime 

 glands, etc., is by no means perfect. Near the posterior end 

 of the body the fin-rays are much more numerous than the 

 myotomes. 



The dorsal fin (Fig. 14, DF) extends through the body region 

 occupied by about the fiftieth to the ninety-fifth (last) myotome. 

 Anteriorly the first three or four fin-rays are very small and 

 scattered, being imbedded in the median dorsal septum just 

 outside the muscles. A little farther back the rays are better 

 developed. Their bases lie in the roof of the neural tube (skel- 

 etogenous layer), and they extend obliquely backward and up- 

 ward through the median dorsal septum to the margin of the 

 fin. The proximal ends of the fin-rays are therefore imbedded 

 between the muscles of the right and left halves of the body, 

 while the distal ends lie between the layers of skin forming the 

 fin fold. The bases of the most anterior rays are free and 

 independent. A little farther back a slender cartilaginous 

 process extends forward in the median line from the base of 

 each fin-ray. Each ray with its corresponding process is at 

 first independent ; but posteriorly the processes become larger 

 and longer, and finally (about seventy-fifth to eightieth myo- 

 tome) they fuse together with the bases of the fin-rays to form 

 the longitudinal median dorsal bar (Fig. 14, MD). This bar is 

 slender anteriorly and triangular in cross-section, but posteri- 

 orly it passes around the end of the tail into the vertical plate 

 (see PI. XXIII, Fig. 15, MD). The median dorsal bar extends 

 along the roof of the neural canal. To it are fused twenty-five 

 to forty fin-rays. (Total number of fin-rays in dorsal fin, fifty 

 to sixty.) In some cases small extra fin-rays are found in the 



