356 Mr. A. S. Woodward on the 



aspect shown in fig. la. The reticular lines already- 

 described on the base of the leaf-shaped plates are here most 

 prominent in a transverse direction, and pass into a remark- 

 able cluster of vermiculating fibres on the median longitudinal 

 ridge. This ridge does not extend to the ends of the plate ; 

 and at the end opposite to that where the bony process 

 appears the ridge terminates at the apex of a bilaterally- 

 symmetrical triangular area, on which the reticular markings 

 exhibit chiefly a divergent fan-shaped arrangement. Another 

 imperfect abraded specimen of the same plate (Brit. Mus. 

 no. 39091), exposed from its aboral face, shows that the 

 terminal bony bar is a tolerably thin lamina directly con- 

 tinuous with the bony base and apparently similar to the 

 latter in texture (fig. 3) . 



There is much variation in the contour of the plates of 

 both kinds commonly referred to Plethodus expansus, but it 

 seems best at present not to separate them under distinctive 

 names. Perhaps the most striking variation is observed in 

 the convex plate from the Gault shown of two thirds the 

 natural size in PI. XIII. fig. 4. Both ends of this fossil are 

 comparatively truncated and the sides are nearly parallel ; 

 but in general aspect it closely agrees with the typical form 

 from the Cambridge Greensand represented in PI. XIII. 

 fig. 1. 



Some small convex plates from the English Lower Chalk, 

 however, seem to be quite distinct from P. expansus, and it 

 is now proposed to describe them under the new specific name 

 of P. pentagon. The type specimen (PI. XIII. fig. 5) from 

 the Lower Chalk of Burham, Kent, is imperfect at one angle, 

 where the section displays the dentinal structure characteristic 

 of the genus. The plate is longer than broad, pentagonal in 

 outline, pointed at one end, truncated and slightly excavated 

 at the other end. The abraded oral face, with punctate 

 markings, is very gently convex and slightly curved upwards 

 at the two sharp angles bounding the truncated end. Three 

 pits or depressions occur on the oral face near this end. The 

 steep lateral border is not tuberculated. A fragment of the 

 pointed end, also from Burham (PI. XIII. fig. 6), bears 

 numerous pits or depressions on the attenuated point. Much- 

 abraded specimens from the Lower Chalk of Dover (Brit. 

 Mus. no. 35874) and Lewes (Brit. Mus. no. P. 2693) are 

 only of interest as extending the known distribution of the 

 species. 



The Lower Chalk also yields a concave dental plate 

 which might serve very well for the opposing dentition of 

 P. pentagon, though, as only detached specimens have been 



