624 L. C. MIA.LL ON THE STRUCTURE OF THE SKULL OE RHIZODTTS. 



45. On the Structure of the Skull of Rhizodus. 

 By L. C. Miall, Esq. (Read June 9, 1875.) 



Much yet remains to be known of the carboniferous genus Rhizodus, 

 of which only the teeth, scales, pectoral arch, jugular plates and 

 mandible have hitherto been identified. 



A large and tolerably perfect skull in the Woodwardian Museum 

 at Cambridge affords an opportunity of a more extended notice than 

 has hitherto been possible. The fossil is derived from coal-shale at 

 Gilmerton, near Edinburgh. The skull itself is largely infiltrated 

 with carbonate of iron. In its present slightly disturbed state it 

 measures about 10 inches by 8 inches. The left orbit and the left 

 nasal foramen are capable of identification. Two teeth are seen. 

 They present the double cutting-edge and the folded base charac- 

 teristic of the genus Rhizodus, and differ in no respect from the 

 teeth of R. Hibberti, Ag. The rami of the mandible have been 

 pressed upwards, so as to conceal the maxillary region. The ex- 

 posed surface of all the bones is covered with a well-defined, coarsely 

 granular sculpture, consisting of ridges and tubercles separated by 

 grooves and pits. Many fractures are apparent ; and the original 

 surface is in some places removed ; so that the delineation of the 

 sutures is often a matter of difficulty. The membrane bones which 

 roof-in the skulls of fishes are so extremely variable, especially in 

 the lateral and posterior tracts, that their homology is often most 

 obscure. In this notice the nomenclature of Prof. Huxley's essay 

 on the Systematic Arrangement of the Eishes of the Devonian 

 Epoch * has been employed. 



The fore part of the skull in its present state shows an unpaired ossi- 

 fication corresponding to the ethmoid of Polypterus, Megalichthys, and 

 other Ganoid fishes (Eth. in fig.). This is bounded behind by a nearly 

 straight transverse suture, with a short median process directed 

 backwards between the anterior ends of the frontal bones. A nasal 

 foramen (Ne), small and apparently round, lies on the left side of the 

 ethmoid, in the angle between it and what is apparently the anterior 

 end of the maxilla. The prsemaxillae are entirely obscured by the 

 mandible. Behind the ethmoid, in the middle line, is a pair of ossifi- 

 cations, which appear to represent the frontals (Fr). They are small, 

 somewhat unsymmetrical, perhaps owing to distortion, and broader in 

 front than behind. External to these, on the left side of the skull, 

 one of the orbits (Orb) can be distinguished. Its borders are frac- 

 tured ; and a mandibular tooth, with the point upwards, lies in it. 

 The cavity appears to have been very small, and of irregular figure, 

 as in Megalichthys. A pair of large parietals (Pa) succeed. The pos- 

 terior boundaries of these cannot be defined. External to the parie- 

 tals there are, as in Megalichthys, a pair of ossifications, which 

 cannot be certainly identified. The occipital region is very obscure, 

 and disfigured by a wide central crack. The postero-lateral ossifi- 

 * Mem. Geol. Survey, Decade x. (1861). 



