8 FOSSIL FISHES OF THE ENGLISH CHALK. 



quadrate ; while the thin, arched, and tapering hinder portion of the ectopterygoid 

 (pcpt.) borders the quadrate anteriorly and the nietapterygoid inferiorly. As 

 shown l)y B. M., no. P. 5()8o, the lower part of the inner face of the ectopterygoid 

 bears a cluster of minute teeth. The premaxilla (figs. 1, 2, prn.v.) completely 

 excludes the maxilla from the upper border of the mouth on each side. It is 

 remarkable for the great relative size of its anterior ascending process, which fits 

 into the deep groove on the anterior face of the mesethmoid. It is constricted, 

 though a little thickened, immediately behind the l)ase of this process ; and its oral 

 face, when Avell preserved {''■(/., B. M., no. P. 568o), ])ears a l)road cluster of minute 

 teeth. The maxilla (figs. 1, 2, d).v.) is a slender l)ar for the greater part of its 

 length, with a large upturned anterior end, which is partly shown in fig. 1, liut 

 better preserved on the left side of the original of fig. 2. It terminates behind in 

 a considerable laminar expansion which is broken and incomplete in the original 

 of fig. 2, but a little less fractured in that of fig. 1. AVhen quite complete, this 

 expansion is slightly deeper than in the latter, and its outer face is not ornamented. 

 The maxilla is overlapped for the greater part of its length by a relatively 

 enormous supramaxilla (figs. 1, 2, sm,v.2), of which the form is l)est shown in 

 fig. 2. It seems to have l)een slightly convex, and it is covered with a very fine 

 rugose ornament, which is often partly destroyed in fossilisation. Below its 

 anterior pointed prolongation there is also a diminutive second supramaxilla 

 (fig. 2, siii.v.l), which is similarly ornamented. The mandible is more or less 

 broken in all the known specimens, but its general shape is indicated in fig. 1. 

 It is short and deep, with a truncated symphysis, and the oral border gradually 

 rising to the coronoid region, of which the highest point is just in front of the 

 articulation for the quadrate lione. The lower portion of the mandil)le is much 

 bent imvards, so that its complete depth is not seen in direct side-view (figs. ], 2). 

 The dentary (*/.) is quite smooth, but the articulo-angular {<i(j.) exhibits a horizon- 

 tally-extended flattened ridge immediately below and in front of the articulation, 

 covered with a fine rugose ornament. Below this ridge there is a deep groove, 

 Avhich may have been occupied by the slime-apparatus of the sensory canal. The 

 thickened oral margin of the dentary bears chistered minute teeth like those of the 

 premaxilla. 



The cheek is covered partly l)y circumorbital plates, partly by scales. Most 

 of the circumorbital plates (figs. 1, 2, ra.) are small, and form a very narrow rim 

 round the orlnt ; but the foremost element of the series, which may be termed 

 antorbital (nn.), is excessively expanded. All these plates are covered with a very 

 fine rugose ornament, and the sensory canal which traverses them is marked by a 

 deep groove. The original outline of the large antorbital plate is best shown in 

 fig. 2, where it is only destroyed by an accidental indent near the upper end of 

 its hinder margin. Its niaxiinuiii (lej)th somewhat exceeds its width, and about 

 equals the vertical diameter of the oi'bit. Its longer axis is slightly inclined 



