110 FOSSIL FISHLS OF TILE ENGLISH CHALK. 



oral lace is smooth, not abraded, but it is deeply pitted at the lateral borders, 

 which curve downwards to the coarsely tuberculated margin (fig. 8 a). 



Horizon and Localities. — Zone of Holaster subglobosus : Burham and Dover, 



Kent ; Lewes, Sussex ; Dorking-, Surrey. 



3. Plethodus oblongus, Dixon. Plate XXII, figs. 9, 10. 



1850. Plethodus oblongus, F. Dixon, Geo]. Sussex, p. 366, pi. xxxii*, fig. 4. 



1899. Plethodus oblongus, A. S. Woodward, Ann. Mag. Nat. Hist. [7], vol. iii, p. 357, pi. xiv. 



1901. Plethodus oblongus, A. S. Woodward, Catal. Foss. Fishes B. M., pt. iv, p. 83. 



Type. — Lower dental plate probably from zone of Holaster subglobosus ; 

 Brio-hton Museum. 



Specific Characters. — A very small species, the low r er dental plate about 2 cm. 

 in length, elongated and irregularly ovoid in shape, the broader end gently 

 rounded, the narrower end truncated; grinding surface of this plate slightly 

 convex. 



Description of Specimens. — A small skull in the Brighton Museum (PL XXII, 

 figs. 9, 9ft), from the same locality as the type specimen, exhibits the rounded 

 end of a similar dental plate (fig. 9 a, I.), while a more satisfactorily preserved 

 skull in the British Museum (PL XXII, fig. 10) evidently belongs to the same 

 species. Part of the cranial osteology of P. oblongus can therefore be determined. 



The cranium is much laterally compressed and about as deep as long, with a 

 very steep frontal profile. The roof -bones have a peculiar fibrous and punctate 

 structure, and some of the sutures between them are distinguishable. Two small 

 plates posteriorly seem to be part of a supratemporal series (fig. 10, s.L). There 

 is a doubtful indication of a small supraoccipital crest (fig. 9, x.), in front of which 

 the transversely-elongated parietals (pa.) taper a little to their meeting in the 

 middle line. External to the parietals and supraoccipital there is a nearly square 

 squamosal (fig. 9, sq.), above the articulation for the hyomandibular (hm.) The 

 relatively large frontal bones (fr.) appear to be fused together, and are not clearly 

 distinct from the mesethmoid. Postero-laterally, where in contact with the hinder 

 elements, the external surface of the frontal region exhibits radiating fibres, but 

 further forwards it becomes finely punctate and rugose. The upper part of the 

 frontal profile is compressed to a sharp edge, but further down it displays a slight 

 flattened concavity (fig. 10 ft). A small depression on each side just in front of 

 the parietal border seems to represent a mucus-pit (m.). The outer border of each 

 frontal is overlapped by two plates (fig. 10, co.), which may be referred to the 

 circumorbital ring. The snout terminates in a thickened and obtusely pointed 

 mesethmoid (eth.), which is ornamented with close reticulating ridges and lines of 



