Cretaceous Fish Plethodus. 357 



discovered, its determination must still remain uncertain. The 

 best plate of this form in the British Museum is shown of the 

 natural size in PI. XIII. fii^s, 7, la. It is strongly arched 

 transversely, less so longitudinally, and a broken section at 

 one end shows the characteri.stic structure. The oral face is 

 not abraded, and is thus not punctate ; it is deeply pitted at 

 the lateral borders, which curve downwards to the coarsely 

 tuberculated mar<2;in. 



It may be observed that a somewhat similar form of dental 

 plate, to be regarded as representing- an allied species, 

 P.furcatus, is known from the Turonian of Bohemia f. 



A fourth form of dental plate referable to Plethodus is of 

 comparatively small size, only 0'021 m. in length and 0"013 m. 

 in maximum width. It is a unique specimen from the Lower 

 Chalk of Clayton, Sussex, now in the Brighton Museum 

 (Willett Coll. no. 153), and is described by Dixon under the 

 nume of P. oblongus [op. cit. p. 366, pi. xxxii.* fig. 4). It 

 is gently rounded at its wider end, almost truncated at the 

 narrower end, and the whole of the oral face is gently convex. 

 This specimen is of great interest because it seems to be 

 closely similar to the rounded end of a thick dental plate 

 mingled with the remains of a small problematical skull trora 

 the same pit, also in the Brighton Museum (Willett Coll. 

 no. 154). In fact there can be little doubt that the latter 

 fossil is referable to Plethodus^ whether its determination as 

 P. ohlongus be right or wrong. There is thus at last some 

 clue to the nature of the fish to which the dental plates under 

 consideration belong. It is only strange that the first evidence 

 as to the characters of the skull should appear in connexion 

 with the smallest species, and not in association with the 

 larger plates, which must iiave been connected with a very 

 massive bony skeleton. 



The imperfect skull in question is shown of the natural 

 size from both sides in PI. XIV. figs. 1, 1 a. The cranium 

 is much laterally compressed and about as deep as long, with 

 a very steep frontal profile. At first sight, indeed, it exhibits 

 much resemblance to that of the extinct Pycnodont fishes. 

 The cranial roof-bones have a peculiar fibrous and punctate 

 structure, and some of the sutures between them are distin- 

 guishable on the right (fig. 1). At the postero-lateral angle 

 above the hyomandibular (hm.) the squamosal element is 

 distinct («$'.), but its upper limit posteriorly is not quite clear. 

 Adjoining the anterior half of its upper border there is a 



t Chimcera furcata, A. Fritsch, * Reptilien uud Fische der bohmischen 

 Kreidef ormation ' (1878), p. 16, woodcut. Referred to Plethodm by 

 A. S. Woodward, Proc. Ueol. Assoc, vol. x. (1888) p. .33], 



