228 PERMIAN FOSSILS. 
tions of the Geological Society ;’’ his determination, therefore, of the species was entirely 
founded on the representation there given. The anterior parts of the fish are wanting, 
but the remainder is well preserved, and the accuracy cof the drawing has enabled 
him to seize upon the most striking specific characters with his accustomed skill. 
Professor King has submitted to me a most beautiful and perfect specimen of this fish 
(vide Pl. XX VI), found at Ferry Hill by Mr. John Jameson of Newcastle, with a 
request that I would complete the specific description carried by Agassiz as far as 
the materials he was cognisant of would allow. ‘This specimen is one of paramount 
importance ; inasmuch as it reveals, for the first time, the true dentition of the genus, 
the knowledge of which necessitates the removal of Platysomus from the family of 
the Lepidoids to that of the Pycnodonts. The outline of the fore-part of the fish, 
from the insertion of the dorsal fin to the occipital crest (deficient in the Hast Thickley 
specimen), is more elegantly curved than in P. s¢rzatus, but from this point it is nearly 
perpendicular until it meets the base of the acute angle formed by the projecting 
jaws. ‘This configuration is very similar to that of the corresponding parts in 
Pycnodus platessus. The upper jaw is nearly in its proper position with reference to 
the lower jaw; but it is rather crushed. The dentary portion of the lower jaw is com- 
posed of a dense triangular bone, very similar to the Pycnodont jaws found at Stones- 
field. Being slightly inclined to one side, two rows of teeth are brought into view, 
the outer one containing eight or nine teeth, the inner one five, full double the size of 
the former. According to the formula given by Agassiz for the Genus Pycnodus, this 
fish had, in all probability, five rows of teeth im each ramus of the lower jaw. The 
form of the teeth is very singular. Count Miinster has described a Pycnodont jaw 
found by Herr Althaus in the Kupferschiefer of Riechelsdorf, named Globulodus 
elegans,’ which is furnished with teeth very similar to those in the present specimen ; 
but the outer row alone is visible. In alluding to this genus of Count Minster, 
Agassiz says,’ that it is probably founded on the dentition of Platysomus, a surmise 
which proves to be perfectly correct. The genus G/obulodus must therefore be 
cancelled. The teeth of Platysomus are clavate in form; a circular crown, with a 
flattened grinding surface, being mounted on a pedicle of much less diameter, the 
decrease in size being effected suddenly by a deep constriction immediately below the 
crown. A fine sulcus circumscribes the triturating surface, apparently indicating the 
point of junction between the harder material of the tooth and the softer substance 
composing the base. No incisor teeth are visible; but it is probable from the promi- 
nence of the anterior angles of the jaws, that they were furnished with teeth of 
a more elongated form than those composing the masticatory apparatus. This 
arrangement of the dental machinery is a sufficient warrant for the removal of 
1 Trans. Geol. Soc., 2d series, vol. 11, pl. xii, fig. 1. 
2 Beitrage, &c., part v, p. 47, plate xv, fig. 7. 
3 Poissons Fossiles, vol. ii, part 1, p. 203. 
