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 of 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 XXVI), 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 ; inasm.uch 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 East Thickley 

 specimen), is more elegantly curved than in P. striatus, 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 

 Pi/cnodus 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 Fycngdus, this 

 fish had, in all probability, five rows of teeth in 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 Glohulodus 

 eleyans,^ 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 Miinster, 

 Agassiz says,^ that it is probably founded on the dentition of Platysomus, a surmise 

 which proves to be perfectly correct. The genus Glohidodus must therefore be 

 cancelled. The teeth of Plati/somus 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 aiiterior 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 



^ Trans. Geol. Soc, 2d series, vol. iii, pi. xii, fig. 1. 

 2 Beitrage, &c., part v, p. 47, plate xv, fig. 7. 

 ^ Poissons Fossiles, vol. ii, part ii, p. 203. 



