114 FOSSIL FISHES OF THE ENGLISH CHALK. 



projected slightly beyond the mandible, shows a tendency to curve upwards in the 

 original of fig. 2. The teeth on each margin are irregularly spaced, are largest 

 above the hinder end of the presymphysial bone, and gradually decrease in size 

 forwards until they disappear at some distance from the end of the snout. They 

 are hollow slender cones, generally more or less recurved, coarsely crimped at the 

 base, vertically striated, and capped by a sharp point of smooth translucent 

 enamel (fig. 2 a). They are directly anchylosed with the bone of the jaw, not in 

 sockets. Behind the rostrum, and opposed to the splenial dentition, is another 

 bone (fig. 1, x) bearing shorter and stouter teeth of the same kind arranged in a 

 very close series. 



The best-known specimen of the mandible is shown from above in PI. XXX, 

 fig. 13, partly from the right side in fig. 3 a. The two rami occupy only one half 

 the entire length of the jaw, the anterior half being formed by the enormously 

 elongated presymphysial bone. Each ramus is laterally compressed and deep, 

 gradually tapering in front; and the coronoid region rises immediately behind 

 the posterior end of the tooth-bearing portion. The two rami meet in front 

 in a very acute angle, and the symphysis is elongate, gradually diminishing 

 to a thin edge below, and thus forming a sloping triangular surface for the 

 overlap and articulation of the large presymphysial bone (see also figs. 1, 4). 

 Anteriorly, for a very short space, the tooth-bearing margin is evidently 

 formed by the dentary element (//.), but the greater part of this margin 

 is occupied by the splenial (spl.), which gradually widens backwards. The 

 presymphysial bone (ps.) is a median, bilaterally-symmetrical element, very 

 gradually tapering to a point anteriorly. As shown in cross-section (fig. 4«), it 

 is hollow, compressed below into a sharp keel, and marked above by a shallow 

 longitudinal channel. All the bones are ornamented, like the rostrum, with 

 delicate longitudinal grooves and ridges, which tend to be most conspicuous near 

 the ventral keel of the presymphysial bone. The only large teeth in the mandible 

 form a single, widely-spaced series in the groove of the presymphysial bone. They 

 are larger than those of the rostrum and are not recurved ; but they are similarly 

 hollow cones, with a smooth translucent tip of enamel, finely striated sides, and a 

 crimped base anchylosed with the supporting bone (figs. 4b, 5). The successional 

 teeth are developed in the spaces between the actually functional teeth, so that the 

 series is always more or less irregular. Very small teeth of the same kind, but 

 stouter, are clustered along each lateral margin of the presymphysial bone, those 

 of one inner series being slightly larger and more regular than the others. These 

 marginal teeth are continued on the dentary, where it enters the oral border, and 

 gradually pass into the splenial dentition, which is mainly adapted for crushing. 

 Minute teeth extend far downwards on the inner side of the splenial, while larger 

 mammilliform teeth are clustered in a pavement on the functional surface (figs. 

 3, 3 I). 



