130 



FOSSIL FISHES OF THE ENGLISH CHALK. 



hinder part of the eye, where their outer portion appears to be fused with the 

 downwardly tapering postfrontal or splienotic (pff-)- The cheek (fig. 1) is com- 

 pletely covered with thin plates, and the rather small eye has an ossified sclerotic. 

 The postorbital plates (}><>■), three or four in number, are much elongated antero- 

 posteriorly, and are traversed near the orbital border by the usual sensory canal, 

 which is filled in the fossil with oxide of iron. There is a narrow suborbital plate 

 (or series of plates), also traversed by the sensory canal (so.) ; and there is a short 

 and deep antorbital (ao.), which appears to be thicker and less flattened than the 

 others. A narrow plate which tapers to a point in front, slightly overlaps the 

 hinder part of the maxilla and may probably be regarded as a supramaxilla. 

 The characteristic maxilla (mx.), with its flattened outer face and nearly straight 

 oral border, is also well seen, though imperfect behind and slightly overlapped 

 above by the cheek-plates. Its long and stout antero-internal process is exposed by 



pmx 



-y-smx. 



Fig. 39. — Pachyrhizodus subulidetis (Owen) ; restored drawing of head, left lateral and upper 

 aspects, about one half natural size. — English Chalk, aij., angular ; ao., antorbital ; d., dentary ; 

 epo., epiotic ; eth., mesethmoid ; fr., frontal ; mx., maxilla ; orb., orbit ; pa., parietal ; pmx., pre- 

 maxilla ; po., postorbitals ; qu., quadrate ; smx., supramaxilla ; so., suborbital ; socc, supraoccipital ; 

 sq., squamosal ; x., upper cheek-plate. 



the accidental removal of the premaxilla. Its inner side is shown by another 

 specimen (PI. XXVII, fig. 3), in which the slender and slightly curved teeth are 

 clearly flanked outside by a low thin wall of bone. Remains of the premaxilla 

 occur in front of the maxilla just mentioned, showing the bases of the usual two large 

 inner teeth, and suggesting that the bone was expanded into a deep lamina which 

 occupied more of the face than the premaxilla in the preceding three species. The 

 mandible occurs in position in the head already described (PI. XXVII, figs. 1, 1 b), 

 but it is so much fractured that the characters of the dentary are better shown in 

 the original of PI. XXVII, fig. 3 a. The symphysis tapers and is much thickened, 

 but does not bear any teeth within the outer series. The outer face of the bone is 

 remarkably smooth, and the longitudinal bulging of its middle portion is especially 

 well seen in the right ramus (fig. 3 a), which is complete below. The teeth are very 

 large compared with those of the maxilla, and their polished crowns curve backwards 

 and inwards. Of the opercular apparatus it can only be said that the preoper- 

 culum (PI. XXVII, fig. 1, pop.) is much expanded at the angle and apparently 



