Cretaceous Fish Plethodus. 359 



the mandible on the left side there is displayed one end of a 

 comparatively thick convex dental plate (h), which has pre- 

 cisely the aspect of that of Plethodus both outwardly and in 

 transverse section. It is not fixed upon bone, and may well 

 have belonged to the hyoid arch. On the left side the preoper- 

 culum {p.02J.) is shown to be very large and widely expanded 

 at its angle. The operculum {op.) is only represented by a 

 fragment. 



A second example of this type of skull, in some respects 

 more satisfactory, is preserved in the British Museum 

 (no. 49895). This fossil, from the Lower Chalk of Dorking, 

 Surrey, displays only the left side of the head, and is shown 

 of the natural size in PI. XIV. fig. 2. The same form of 

 cranium is readily distinguishable ; but the squamosal 

 element is imperfect, the lateral margin of the frontal region 

 is bounded by a postfrontal {pt.fr.) and a prefontal {pr.fr.) 

 membrane-bone, while the mesethmoid terminates in front in 

 a thickened obtusely pointed rostrum (»•.), which is orna- 

 mented with close reticulating ridges and lines of tubercles *. 

 There are also two small plates posteriorly {s.t.) which may, 

 perhaps, be supratemporals. The frontal (//•.) exhibits the 

 mucus-pit (in.) as before, and it seems to cover only the hinder 

 part of the rostral region, though its apparent boundaries in 

 the fossil are probably deceptive and due to accidental cracks. 

 The mesethmoidal septum (in.eth.) is seen to meet a vertical 

 plate rising from the basicranial axis, and the latter exhibits 

 the same remarkable expansion as in the previous specimen. 

 At least this seems to be the case, for there is evidence of 

 crushing in this region, and a longitudinal ridge along the 

 supposed basicranial expansion is probably the fractured and 

 slightly displaced base of the vertical plate just mentioned. 

 In a plane distinctly external to the expansion there are 

 remains of the hyomandibular {km.) and the pterygo-quadrate 

 arcade; the metapterygoid {m.pt.), quadrate {qu.), and long 

 slender ectopterygoid with its minute teeth {ecpt.) being 

 readily distinguishable. Indeed, if the previous specimens 

 were unknown the expansion itself might readily be inter- 

 preted as entopterygoid, while the ridge upon it would be 

 considered as an accidentally crushed and displaced slender 

 parasphenoid. The maxilla {ma?.) is a deep laminar bone, 

 finely tuberculated at its oral margin, which forms the greater 

 part of the upper border of the mouth. The premaxillae are 



* It may be noted that this rostrum exhibits much resemblance to a 

 comparatively large tip of a snout from the Sussex Chalk, once pro- 

 visionally assigned to on Acipenseroid fish (A. S. Woodward, Proc. 

 Geol. Assoc, vol. xi. 1889, p. 31, pi. i. tig. (3). 



