ELASMODECTES. 191 



Tijpe. — Mandibular dental plates; Brighton Museum. 



Specific, Characters. — The type species, with mandibular dental plate sometimes 

 measuring 2-5 cm. in length, usually smaller. Mandibular plate thickest at tlie 

 symphysis, with a high beak ; prominences for outer tritors very gently sloping 

 backwards, not steep ; the separate points of the tritors round, irregular in size, 

 tending to arrangement in two series in each case. Dorsal fin-spine nearly smooth, 

 but with a tendency towards vertical ribbing, especially near the anterior keel and 

 the base. 



Description of Specimens. — This species is known chiefly by mandibular dental 

 plates, but one specimen (PI. XXXIX, fig. 4) exhibits part of the head and 

 abdominal region with the characteristic dorsal fin-spine. The two mandibular 

 plates are well preserved and shown from the inner face in the type specimen 

 (PJ. XL, fig. 1) ; their outer face is seen in other examples. 



The cartilages of the head and pectoral arch are only superficially calcified, and 

 thus much broken and distorted in the chalk. The calcified layer is always very 

 thin and consists of closely-compacted minute tesseras. The frontal profile of the 

 skull is steep (PI. XXXIX, fig. 4), and in the fossil there are remains of cartilage 

 in advance of the mouth. One of the vomerine dental plates (y.), displaced so as to 

 show its inner concave face, appears to resemble the corresponding plate of 

 Ischijodiis in shape, with nearly parallel anterior and posterior margins, but its 

 oral margin is broken away. The right palatine plate (^?.) is seen only in outer 

 view, and the arrangement of its tritors remains unknown. Of the mandibular 

 plate (md.) the outer face is nearly flat, with slight traces of growth-lines, but no 

 thickening near the oral margin. It is completely shown in PI. XL, figs. 1 — 3. 

 The symphysis is distinctly the thickest part of the dental plate (figs. 1 h, 2) and 

 the symphysial facette (fig. 3 a) is flattened. The post-oral margin is somewhat 

 less steeply inclined than the symphysial margin, and on the wavy oral margin 

 the middle prominence is nearer to the beak than to the hinder prominence. On 

 the inner face (figs. 1, la, 3) the horizontal growth-lines are especially conspicuous, 

 and all the tritors are visible. All the separate points of the tritors are rounded, 

 but they are very variable in size and shape, and although they tend to arrange- 

 ment in double series, they are remarkably irregular. The two series of beak 

 tritors diverge downwards from the apex ; and there is occasionally (fig. 1 a) a 

 row of tritoral points along the lower half of the symphysial margin. The outer 

 tritors extend chiefly along the gentle posterior slope of each marginal prominence, 

 but also occur slightly on the steep anterior slope. In each case the largest points 

 are near the apex. 



The axial skeleton of the trunk is marked by the usual series of calcified rings 

 (PI. XXXIX, figs. 4, 4(6), which originally encircled the notochord. 



The rostral spine is unknown, the head now described being probably that of 

 a female. The dorsal fin-spine (PI. XXXIX, fig. 4, (/.), however, is seen in 



