No. 2.] MORPHOLOGY OF THE MYXINOIDEI. 195 



and below the notochord, forming a complete cartilaginous 

 sheath. The form and extent of this sheath varies consider- 

 ably in different specimens. 



The parachordal cartilage on each side is expanded laterally 

 into a thin sheet of cartilage forming the auditory capsule 

 (PI. XXII, Fig. 4; PI. XXIII, Fig. 7 A). These capsules 

 are two somewhat kidney-shaped structures, with their long 

 axes lying parallel to the parachordals. Their shape in cross- 

 section is shown in Fig. 4. The ventral walls are contin- 

 uous with the inferior external angles of the parachordals. 

 In the anterior and posterior regions of the capsules the 

 dorsal walls are continuous similarly with the superior exter- 

 nal angles of the parachordals ; in the middle region, how- 

 ever, the dorsal walls of the capsules are separated from 

 the parachordals by the auditory foramina (PI. XXII, Fig. 4 ; 

 PI. XXIII, Fig. 7, /). These are two large elliptical foram- 

 ina extending the greater part of the length of the audi- 

 tory capsule. The foramina are closed by the membranous 

 cranial wall, which is perforated by small openings for the exit 

 of the auditory nerves. The posterior third of the auditory 

 capsules is continuous, externally, with the upper ends of the 

 hyoid arches. Anteriorly the walls of the auditory capsules, 

 together with the anterior ends of the parachordals, are fused 

 with the posterior ends of the trabeculae. 



The trabeculae (PI. XXII, Figs. 5, 6; PL XXIII, Fig. 7, 

 TV., tr) are a pair of latero-inferior bars which form a frame 

 for the support for the anterior half of the membranous cra- 

 nium. Posteriorly they are comparatively thick and strong. 

 At their posterior ends they have three attachments : (i) an 

 internal, with the anterior ends of the parachordal cartilages ; 

 (2) a median, with the anterior wall of the auditory capsule ; 

 and (3) an external, with the posterior process from the 

 pterygo-quadrate cartilage. Anteriorly, a short distance be- 

 hind the anterior end of the membranous cranium, the anterior 

 horns of the trabeculae extend downward, forward, and inward, 

 as slender bars {tr) which merge into the median hypophysial 

 plate. At the point of junction of the anterior horn {tr) with 

 the main bar of the trabecula {Tr) there is given off externally, 



