PACHYTHRISSOPS. 131 



supramaxillae, of which the posterior is the larger, and excavated in front for the 

 anterior, which is elongate-triangular in shape. In the mandible, the dentary bone 

 is relatively large, and its posterior end extends along the lower border almost as 

 far as the position of the mandibular articulation. Though well shown in the 

 type specimen, its complete shape is better seen in the original of PI. XXV, fig. 2. 

 The bone is truncated at the symphysis, and gradually rises in its middle portion 

 into a high and stout coronoid process, which inclines a little backwards and ends 

 abruptly behind. Beyond this process the tapering posterior end of the bone is 

 much shorter than in the corresponding part of Leptolepis. The lower half of the 

 bone is bent inwards along an obtuse-angled longitudinal ridge beginning below 

 the middle of the symphysis, and this face is marked by large pits and a groove, 

 which indicate a considerable development of slime-apparatus. The outer face 

 is for the most part smooth, but there is a little rugosity near the oral border, 

 which (as shown in PI. XXIV, fig. 4 a) bears small, hollow, smooth, and bluntly- 

 conical teeth. 



The hyoid arch, seen in PL XXIV, fig. 4, is relatively large, the ceratohyal 

 (c/i.) being laterally compressed, mesially constricted, and deepest behind at its 

 articulation with the epihyal (eph.). Its upper angles seem to be united by a rod 

 of bone as in Leptolepis. The basihyal (bh.) is very short. 



The preoperculum (PI. XXV, fig. 1, pop.) is sharply angulated, with the lower 

 limb nearly as long as the upper limb, and the anterior border much thickened. 

 This thickening is smooth and widest at the bend, from which a few coarse ridges, 

 more or less fused into a reticulation, radiate backwards over the thin triangular 

 expansion. The traversing slime-canal is very large. The operculum {op.) seems 

 to have been two-thirds as wide as deep, and is also thickened along its anterior 

 border, from which at the point of suspension a short ridge diverges at an acute 

 angle downwards and backwards, as if to bound a slime-pit (PI. XXV, fig. 1 a). 

 The outer face is entirely smooth. The suboperculum (sop.) and interoperculum 

 are comparatively small, and both these and the broad upper branchiostegal rays 

 are smooth. The lower branchiostegal rays, of which some are seen below the 

 ceratohyal in PI. XXIV, fig. 4 (br.), are slender, rod-shaped, and spaced. 



As shown in several specimens, but especially well in the original of PI. XXIV, 

 fig. 4, the gill-arches bear a close series of large bony gill-rakers (g.r.) which are 

 smooth, laterally-compressed elongated cones, each with a notch just above its 

 base of attachment. Similar gill-rakers appear to occur in Leptolepis (B.M. no. 

 P. 3674), MhaUon (B.M. no. 37042), and Thrissops (B.M. no. P. 917). 



The vertebras in the type specimen (PI. XXV, fig. 1) are in undisturbed series, 

 but they are somewhat broken, and not so well seen as in some of the portions of 

 larger individuals. About 35 can be counted in the abdominal, 26 in the caudal 

 region. The centra are all well ossified, the primitive double cone being thick and 

 forming two wide rims, between which the secondary ossification is in fine longi- 



