of Amphicentrum granulosum. 277 



margin forming- a thin sharp edge, separated by a considerable 

 groove from the prominent tooth-bearing ridge of the element 

 last described. In the small mandible figured by Dr. Young*, 

 of which the original is now before me, this edge is broken 

 away save quite in front ; but it is well shown, seen from the 

 inner aspect, in the head represented in PI. IX. fig. 2. At 

 the symphysis the anterior extremity of this element is sharp, 

 prominent, and excavated above, and in this manner comes to 

 look somewhat like the counterpart of the premaxillary beak 

 opposed to it. The external surface is smooth and ganoid, 

 being prettily sculptured with short ridges, furrows, and 

 tubercles. 



Other distinct mandibular pieces were certainly present ; 

 of these the only one seen with any distinctness is the angular, 

 a narrow plate at the angle of the jaw, much resembling the 

 corresponding element in the Pala3oniscida3. 



The palatal tooth-bearing plate {pa, PL IX. figs. 9 and 10 

 detached, and figs. 3 & 4 in situ) differs, as might be expected, 

 considerably in form from the mandibular. What may be 

 designated the hody of the bone presents a gently convex 

 oral surface, bearing a large patch of tolerably prominent 

 shining tubercles, some round, some oval, by which the 

 roof of the mouth was roughened and armedf. The supe- 

 rior internal margin is thin ; the external inferior one is 

 convexly curved, and presents two prominent dentigerous 

 ridges, separated by a groove, and coalescing posteriorly. 

 The external of the two ridges is the more prominent ; 

 but the dental tubercles are developed nearly equally on 

 both, being strongest in front and fading aAvay posteriorly 

 into mere undulations ; considerable individual differences 

 are also here observable in the number and degree of promi- 

 nence of these dental eminences. Behind the union of the 

 two dentigerous ridges, this palatopterygoid sends down a 

 short blunt process (x, fig. 9), which seems, most probably 

 along with subjacent cartilage, to have taken a part in the 

 articulation of the mandible, there being no distinct quadrate. 

 Above this articular process the posterior part of the bone 

 projects upwards in a slightly expanded wing or lamina (?/), 

 which, bending also a little outwards, comes into contact with 

 the hyomandibular (km, fig. 3) by its thin hinder margin. 

 The outer surface is concave ; the lower margin, external to 



* Op. cit. pi. xx. fig. 3. 



t These tubercles are probably the same with those referred to by 

 Dr. Young at p. 309 of his paper, and represented in fig. 7, pi. xx. of his 

 illustrations. Their number is not, however, limited to three rows, nor 

 is their arrangement always linear. 



