PACHYTHRISSOPS. 135 



the preoperculum is relatively large, the process (p.) for the suspension of the 

 operculum is much elongated. Just below the latter process the bone is antero- 

 posterior^ compressed and rises to a sharp vertical crest along its outer face. 

 The triangular quadrate (Text-fig. 40, (pi.) is cleft postero-superiorly for the 

 rod-shaped symplectic (PI. XXV, fig. 4, sy.), and articulates above with a large 

 triangular metapterygoid (mpt.). Its articular condyle for the mandible is 

 somewhat constricted from the main part of the bone, and has a robust inwardly- 

 directed process arising from its base. The entopterygoid (enpt.) is an elongated 

 thin lamina of bone, bluntly pointed in front, truncated behind, slightly convex on 

 the oral face, and inclined inwards towards the parasphenoid. The ectopterygoid 

 (PL XXV, fig. 4, ecpt.) is a narrower elongated lamina, more rapidly tapering in 

 front. It is toothless along the lower border, but its oral face, like that of the 

 other pterygoid plates, has not been seen. The premaxilla, imperfectly shown in 

 the type specimen (Text-fig. 40, pmx.), is comparatively small, but longer than 

 deep. The large maxilla (Text-fig. 40 and PI. XXV, fig. 4, mx.) is arched, and 

 its outer face is nearly smooth, marked only in places with a faint rugosity; its 

 convex oral border exhibits the points of attachment of clustered minute teeth. 

 Its posterior two-thirds are overlapped by two supramaxillae, of which the 

 posterior is the larger and excavated in front for the anterior, which is very long 

 and narrow and tapers forwards. The outer face of the posterior supramaxilla is 

 slightly rugose, and bears a ridge along the lower margin of its antero-superior 

 extension. The articulo-angular bone of the mandible (Text-fig. 40, ag.) is short 

 and deep, and its lower portion is ornamented with fine longitudinal rugae. The 

 very large dentary (d.), which is imperfectly known, is evidently truncated at the 

 symphysis, and its lower half is sharply bent inwards, separated from the upper 

 half by an obtuse longitudinal ridge. 



The hyoid arch, partly seen in the original of PL XXV, fig. 4, is relatively 

 large, and the upper branchiostegal rays, attached to the epihyal, are especially 

 large smooth laminae of bone (br.). 



The preoperculum, incomplete and abraded in the type specimen, is best seen 

 in PL XXIV, figs. 6 (pop.), 6 a. It is sharply angulated, with a relatively large 

 lower limb, and a great smooth expansion behind. The anterior border is much 

 thickened and smooth, and from this thickening six or seven coarse ridges radiate 

 over the outer face of the lower limb. The operculum (op.) is imperfect in all the 

 specimens, and the proportions of the suboperculum and interoperculum are 

 uncertain. The operculum seems to have been smooth or only faintly rugose, 

 except at its postero-superior border, where it is finely fimbriated. 



A single vertebral centrum attached to the occipital region of the type 

 specimen is completely ossified, deeper than wide, and very short, marked on the 

 sides by fine striations which extend between a thickened rim anteriorly and 

 posteriorly. The neural and haemal arches are inserted in pits. As shown by the 



