108 FOSSIL FISHES OF THE ENGLISH CHALK. 



where it becomes concave; its truncated border more or less tuberculated. (Jpper 

 dm! al plate nearly as broad as long and regularly concave. 



Eemarhs.— It is curious that of so large a species no bones have hitherto been 

 identified. The skull will probably prove to be very similar to. that of Anogmius 

 aratns and A. ntteli; and it seems likely that the tip of its snout has already been 

 wrongly described as belonging to an Acipenseroid fish. 1 



Description of Specimens. — The type specimen in the Willett Collection, Brighton 

 Museum, is part of the side of a large lower dental plate, which when complete 

 probably resembled the original of PL XXII, fig. 1, in size and shape. Its slightly 

 sinuous but generally convex grinding- surface does not exhibit any punctations, 

 and consists of a thin, yellowish, opaque layer covering the thick agglomeration of 

 parallel vertical tubules, which form the main mass of the plate. There is a base, 

 presumably of bone, beneath this mass, but it seems to be comparatively thin. 

 The lateral border of the plate is somewhat truncated, and it may have borne a few 

 blunt tubercles, but this is not quite certain. 



Fragments of many similar plates are known, but no complete examples have 

 hitherto been discovered in the Chalk. With them occur somewhat concave plates 

 of the same structure, which evidently represent the opposing dentition of the 

 upper jaw. One such specimen, which is remarkably concave and must have been 

 originally about as broad as long, is shown in PI. XXII, fig. 2. Its oral face, 

 being unabraded, 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 a median bony bar (p.) is especially well preserved at one 

 end (presumably the posterior). 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 (x.) of uncertain form. The 

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

 in fig. 2 a. Like the attached face of the lower dental plates, it is marked by 

 very fine reticular lines ; but here the lines are most prominent in a transverse 

 direction and pass into a remarkable cluster of vermiculating fibres on the median 

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

 the end which we regard as anterior it terminates at the apex of a bilaterally- 

 synimetrical triangular area, on which the reticular markings exhibit chiefly a 

 divergent fan-shaped arrangement. Similar markings are seen in another specimen 

 (% 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. They all have the minute structure represented in 

 PI. XXII, figs. 4, 5. The grinding surface, when unworn in the fossils, is covered 



1 A. S. Woodward, "On the Palaeontology of Sturgeons," Proc. Geo!. Assoc., vol xi (1889), p. 

 31, pi. i, fig. 6. 



