No. 2.] MORPHOLOGY OF THE MYXINOIDEI. 21 7 



flattened dorso-ventrally. This plate forms the ventral wall of 

 a pocket in which the posterior end of the large dorsal abdom- 

 inal vein lies, its main trunk terminating here. A similar struc- 

 ture has been described by Cleland for Myxine. In the anterior 

 region the dorsal surface of the median ventral bar is merely in 

 contact with the ventral surface of the notochord. Posteriorly, 

 however, the ventral bar sends up lateral extensions around the 

 sides of the notochord, so that the latter lies in a groove on the 

 dorsal surface of the bar. Near the posterior end these lateral 

 plates extend still farther up around the sides of the neural 

 tube, so that the median ventral bar is F-shaped in cross-sec- 

 tion. The appearance near the end of the notochord is shown 

 in PI. XXIII, Fig. 15. Just behind the plane of the section 

 shown in this figure a cartilaginous band rises on each side 

 from the median ventral bar, and fuses with the median dor- 

 sal bar, formi7ig a complete neural arch aroujid the nem-al tube 

 (PI. XXIII, Fig. 14, r). This arch lies just anterior to the last 

 two myotomes. The posterior end of the neural tube is slightly 

 dilated and surrounded by cartilage, excepting the upper lat- 

 eral portions. The spinal cord itself is not dilated at the 

 posterior end. 



A very thin irregular sheet of cartilage is found in the wall 

 of the cloaca, especially in the anal region (Fig. 14, av). It 

 extends across the cloaca in the postanal septum. The devel- 

 opment of this cloacal cartilage varies a great deal in different 

 specimens. In a few cases it is found well developed, in others 

 only to a slight extent. It is possible that this cartilage serves 

 to expand the anal opening in anal respiration. 



Exoskeleton. 



The only skeletal parts which belong to the integument are 

 the horny teeth. The dorsal tooth (PI. XXII, Fig. 6, dt) is a 

 single corneous conical structure in the roof of the mouth. 

 Its base is imbedded in a firm, disc-shaped matrix which lies 

 in the median line immediately below the palatine commissure 

 {cni). The tooth is light brown in color, thick at the base, but 

 slender toward the end, which curves downward and backward, 



