206 AYERS AND JACKSON. [Vol. XVII. 



{aa) are large, thin, oval sheets of cartilage, broad anteriorly 

 and narrower posteriorly. They are curved slightly so as to 

 be somewhat convex upward and concave downward. Their 

 ventral surfaces rest directly upon the external bars of the 

 anterior segment of the basal plate. The upper surfaces of 

 the two plates are, therefore, correspondingly slightly inclined 

 toward each other in the normal position. The external 

 margins of the plates are smooth and rounded. The internal 

 margins are nearly straight, and approach each other in the 

 median line anteriorly (being separated from each other only 

 by the median piece and anterior processes) but diverge pos- 

 teriorly, being widely separated at the posterior ends. The 

 internal margins are connected with the median piece, as 

 already described. The posterior processes are separated from 

 the anterior, and from the lateral plate, on each side, by a 

 long, narrow slit, parallel to the inner margin of the lateral 

 plate. On account of this slit, the posterior processes, which 

 are long and slender, have the appearance of a third arch 

 closely united with the anterior arch. At the posterier ex- 

 tremity, each lateral plate has two processes, an external {e) 

 and an internal (z). The external process curves around up- 

 ward and inward, and ends freely in a fold of the mucous 

 membrane just behind the inner row of teeth. The internal 

 process curves backward, upward, and inward. It fuses with 

 the end of the lateral bar of the posterior arch. On their 

 dorsal surfaces the lateral plates bear the matrices in which 

 are imbedded the four rows of teeth. The posterior arch 

 (PL XXIII, Figs. lo, II, pa) is composed of two curved flat- 

 tened bars. These bars are rather wide anteriorly, but narrow 

 abruptly as they approach, to fuse with each other in the 

 median line by a short commissure. The anterior half of the 

 arch curves downward to form a convex projection correspond- 

 ing to that formed by the median piece of the anterior arch, 

 and fitting in the median dorsal groove of the basal plate below. 

 The posterior halves of the lateral bars diverge posteriorly and 

 become narrower. Near the posterior end of each there is a 

 slight enlargement, at which the bar turns sharply outward to 

 fuse with the internal process (i) from the lateral plates of the 



