140 FOSSIL FISHES OF THE ENGLISH CHALK. 



the skull, and part of the opercular apparatus. Numerous other skulls and jaws, 

 especially those in the British Museum, are well preserved, but the trunk and fins 

 remain almost unknown. 



The cranium (PL XXIX, figs. 3 — 7) is deeper than wide, somewhat constricted 

 just in front of the postfrontal (sphenotic) prominences, expanded again between 

 (he large orbits, and ending in a very short ethmoidal region. The bones of the 

 cranial roof are a little thickened and of loose texture, without external ornament, 

 and not clearly exhibiting the sutures between the constituent elements. A 

 delicate median vertical supraoccipital crest (figs. 4, 7, cr.) extends forwards to the 

 f rentals (Jr.), which are traversed by a shallow median longitudinal depression. 

 The otic and prefrontal regions are completely ossified. The basicranial axis 

 is almost parallel with the roof of the skull, as shown by the parasphenoid in 

 fig. o. The orbit is remarkably large, occupying nearly the front half of the head, 

 and there are no traces of cheek-plates in any specimen. A small apparently 

 superficial bone, however, hangs downwards and outwards from the hinder part of 

 the outer border of the frontal. This bone (figs. 3, 7, .*', and fig. 8) is smooth, 

 narrow in its lower free half, triangularly expanded at its upper attachment. 



The mandibular suspensorium is nearly vertical, so that the articulation of the 

 mandible is beneath the occiput. The hyomandibular (figs. 5 — 7, hm.) is a much- 

 expanded thin lamina of bone, strengthened by a vertical ridge on its outer face, 

 and with a slight prominence for the support of the operculum. Its narrow 

 truncated lower end appears to be completely behind the quadrate, but a 

 symplectic has not been observed. The triangular quadrate (</n.) is relatively 

 small, and its upper margin seems to be in contact only with the metapterygoid 

 (mpt.), which does not bear any process for articulation with the cranium. The 

 entopterygoid (enpt.) is a large, oval, delicate lamina of bone, without teeth. The 

 ectopterygoid (ecpt.) is a long narrow bar at the lower border of the latter, bearing 

 in its anterior half a single marginal row of conical teeth, which diminish in size 

 backwards and are a little larger and stouter than most of those of the maxilla. 

 On the middle of its inner face (fig. 9) the ectopterygoid also bears a few tubercular 

 teeth. The premaxillas (figs. 4 — 7, pmx., and fig. 10) are very stout and almost, if 

 not completely, fused together in the median line, besides being firmly fixed to the 

 ethmoid. Each bears a close series of four large teeth, Avhich slightly diminish 

 in size backwards ; and the bone is somewhat constricted above the dentigerous 

 border, though the ascending portion is relatively wide and pierced by an oval 

 foramen for the passage of the olfactory nerve (fig. 4). The maxilla (figs. 4 — 7, 

 mx.) tapers to a slender portion in front, and is truncated, with a more or less 

 jagged border, in its expanded portion behind. Its flat outer face exhibits a 

 coarsely fibrous structure, and is rarely tuberculated (fig. 11) ; its slightly sinuous 

 oral border bears a single close series of relatively small and slender teeth, which 

 dimmish in size backwards. If a supramaxilla was present, it must have been 



