PLETHODUS. 109 



with a thin, dense layer, which is almost opaque in microscope-sections, showing 

 only a granular structure, no organic tissue. Below this the main mass of the 

 plate consists of a dense cluster of vertical tubules, which are conspicuous in 

 sections even to the naked eye (fig. 4). Examined in transverse section under the 

 microscope (fig. 5), each tubule is seen to consist of several concentric layers, 

 which are variously and unequally stained in the fossils; and these layers are 

 crossed by very minute, more or less wavy tubuli, which radiate from the central 

 canal. The base of the plate is formed of cancellous true bone, which often 

 penetrates the lower part of the overlying layer in slender processes between the 

 tubuli. 



Horizons and Localities. — Turonian zones : neighbourhood of Lewes. Zone of 

 Holaster subglobosns .• Hailing and Burham, Kent ; Glynde, Clayton, and Newtimber, 

 Sussex. Zone of Terebratulina gracilis : Cuxton, Kent. Undetermined zone : 

 South Wiltshire. Also ranging downwards to Upper Grreensand and Gault. 



2. Plethodus pentagon, A. S. Woodward. Plate XXII, figs. G — 8. 



1899. Plethodus pentagon, A. S. Woodward, Arm. Mag. Nat. Hist. [7], vol. iii, p. 356, pi. xiii, 

 figs. 5—7. 



1900. Thryptodus sp., F. B. Looniis, Palseontogr., vol. xlvi, p. 235. 



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



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



Specific Characters. — A smaller species than the type, the lower dental plate 

 attaining a length of about G cm., with a maximum breadth of 4 cm. Lower dental 

 plate elongated, pentagonal in shape, widest near the pointed end, and the border 

 of the truncated end slightly excavated ; grinding surface slightly convex. 

 Supposed upper dental plate quadrangular. 



Description of Specimens. — The type specimen (PI. XXII, fig. G) is only 

 imperfect at one angle, where the section displays the dentinal structure charac- 

 teristic of the genus (fig. G a). Its oral face is abraded, and thus exhibits punctate 

 markings due to the exposure of the ends of the constituent tubules (fig. G b) : 

 it is very gently convex, and slightly curved upwards at the two sharp angles 

 which bound the truncated end. Three pits or depressions occur on the oral face 

 near this end. The steep lateral border is not tuberculated. Part of the pointed 

 end of a similar plate (fig. 7) bears numerous pits or depressions on the attenuated 

 point. 



A concave dental plate which probably represents the upper or opposing 

 dentition of P. 'pentagon, is shown in PI. XXII, fig. 8. It is oblong and quad- 

 rangular in shape, strongly arched transversely, and less so longitudinally. Its 



