204 AYERS AND JACKSON. [Vol. XVII. 



anterior segment, with the cartilages of which it is connected 

 by a slightly movable joint. The middle segment is made up 

 of a pair of flattened plates, large, thick, and strong, which are 

 closely united in the median line, excepting at the anterior 

 end (PL XXIII, Figs. 8, 9, Me). In the anterior fifth, the 

 plates diverge laterally and enclose the posterior half of the 

 diamond-shaped space which extends forward between the pos- 

 terior ends of the inner bars of the anterior segment. At the 

 postero-external angle on each side a short cylindrical process 

 (PI. XXIII, Figs. 7, 8, sb) is given off which extends upward, 

 outward, and backward to fuse with the lower ends of the first 

 two branchial arches (<5r, and br^. The plates of the median 

 segment are nearly rectangular in outline, thicker externally 

 than internally, and curved so as to be convex ventrally and 

 concave dorsally. They are about as long as the cartilages of 

 the anterior segment and as wide as both anterior pairs com- 

 bined. They are inclined so as to form a continuation of the 

 trough described in the anterior segment, the chief difference 

 being that the trough is narrower and deeper in the middle seg- 

 ment. Moreover, each of the plates of the middle segment 

 has two longitudinal grooves on its dorsal surface. The first 

 is along the internal margin and unites with that of the oppo- 

 site plate to form a median groove which transmits the main 

 division of the M. retractor mandibidi tendon. The second 

 groove is along the median dorsal surface, and serves to trans- 

 mit, on each side, the lateral division of the same tendon (see 

 PI. XXIII, Figs. 10, II, t)} Anteriorly, as already described, 

 the middle segment articulates with the cartilages of the ante- 

 rior segment. Posteriorly it is immovably attached to the 

 third segment, ^". 



The third segment, B'\ forms the posterior division of the 

 basal plate, and is considerably longer than both the first and 

 second segments combined. It is a heavy, unpaired structure, 

 corresponding in width to the middle segment anteriorly, 



1 It is worthy of mention that on the ventral surface the cartilages of the 

 median segments show distinct longitudinal markings on each side, which may be 

 obsolescent sutures indicating that the pair of plates is composed of two fused 

 pairs, corresponding to the two pairs of the anterior segment. 



