Cretaceous Fish Plethodus. 355 



The more abraded and imperfect specimens of the same 

 dental plate from the Cambridge Greensand (e. g. Brit. Mus. 

 no. P. 7274) exhibit a distinctly punctate crown, the puncta- 

 tions usually bordered by a raised rim. These evidently 

 depend upon the structure of the thin, opaque, superficial 

 layer of the tooth, but are only evident when the outermost 

 surface is destroyed. 



The thick translucent layer of vertical dentine-tubules is 

 always distinct in broken sections of the plates; and when 

 the bony base is also preserved, this is seen to have a verti- 

 cally fibrous structure curiously resembling that of the 

 dentine, but much finer. Sometimes, indeed {e.g. Brit. Mus. 

 no. 39103), appearances have been mistaken to indicate two 

 dental plates, one resting on the other. The fibrous bony 

 base is, in fact, nearly as thick as the crown, and there is a 

 sharp plane of demarcation between these two parts, which 

 are sometimes not completely in contact, but exhibit a small 

 interposed cavity. 



At the same horizons as these slightly convex leaf-shaped 

 dental plates there occur somewhat concave plates of similar 

 structure, which seem to represent the opposing dentition. 

 They are, indeed, commonly labelled Plethodus expansus in 

 collections. An imperfect specimen from the Lower Chalk 

 of Glynde in the Brighton Museum (Willett Coll. no. 152) 

 is broken across, exposing the characteristic dentine and basal 

 bone, while its abraded oral face is punctate. It appears as 

 if it had been bilaterally symmetrical, and from the middle of 

 one end of the base there projects part of a fibrous azygous 

 bony bar for an attachment of some kind. The bar is not 

 seen at the other end, where, however, there is some indica- 

 tion of a paired connexion with adjoining bones. A more 

 fragmentary specimen of the same plate with incomplete 

 border is shown of two thirds the natural size in PI. XIII. 

 figs. 2, 2 a, 2 b. This was obtained from the Lower Chalk 

 of Kent, and is now in the British Museum. It is remarkably 

 concave and must have been originally about as broad as 

 long. Its oral face (fig. 2) is not punctate, but the marginal 

 area is covered irregularly with numerous shallow pits. 

 The truncated border (fig. 2 b) is tuberculated, as in the leaf- 

 shaped plates, and the median bony bar (p) at one end, noted 

 in the previous specimen, is especially well preserved, though 

 still incomplete. The form and direction of this bar are 

 shown in figs. 2, 2 b, while adjoining it on each side in a 

 nearly parallel plane there are remains of a comparatively 

 thin lamina of bone (#) of uncertain form. The attached 

 face of the dental plate, so far as exposed, has the curious 



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