No. 2.] MORPHOLOGY OF THE MYXINOIDEI. 205 



but tapering to a point posteriorly. It has a deep dorsal 

 groove corresponding to that formed by the anterior segments, 

 and transmitting the tendon of the M. retractor maiidibuli. 

 Consequently it is ^-shaped in cross-section. Posteriorly this 

 groove flattens out, and the third segment of the basal plate 

 becomes continuous dorsally and laterally with the fibers of 

 the co7istrictor niusadi mandibiili muscle. It is quite evi- 

 dent that the third segment is not a true cartilage, but is 

 formed by a chondroidal modification of the tendon of the 

 "constrictor" muscle. It is white in color when preserved in 

 formalin, contrasting strongly with the reddish-colored anterior 

 cartilaginous segments. A histological examination shows it to 

 be made up of a very peculiar tissue, more nearly related to 

 notochordal tissue than to the cartilage found in the other 

 skeletal parts. 



The dental plate (PI. XXII, Fig. 6, D. \ PL XXIII, Figs. 

 10, 11) is a cartilaginous structure resting upon the anterior 

 segment of the basal plate and supporting the four rows of 

 horny teeth. It is a flattened framework, consisting of two 

 arches, an anterior and a posterior. The anterior arch is made 

 up of a median piece (PI. XXIII, Fig. 11, Md), two lateral 

 plates {aa), and two pairs of connecting processes (r, v). 



The median piece is a thin elliptical plate, with its long axis 

 in the median line. It is curved from side to side, the down- 

 ward convexity fitting into the dorsal groove of the anterior 

 segment of the basal plate. The anterior end is somewhat 

 depressed where it passes into the protractor tendon. 



The anterior half of the median piece extends farther for- 

 ward than the anterior margins of the lateral plates. Connect- 

 ing the median piece with the lateral plates are two pairs of 

 lateral processes (Fig. 11, r, v). The anterior processes if) are 

 short and are given off from the middle of the median piece 

 on each side, and extend backward, outward, and slightly 

 upward to the antero-internal angles of the corresponding 

 lateral plates. The posterior processes {v) are longer. They 

 extend from the posterior angles of the median piece on each 

 side, backward, outward, and upward to join the lateral plates 

 about the middle of their internal margin. The lateral plates 



