354 Messrs. Hancock & Attliey on Reptile- and Fish-Remains 



A fragment of a maxillary bone of Rhizodopsis has, it is im- 

 possible to doubt, served for the establishment of the so-called 

 Characodus (pi. 13). Here there is not one tooth left ; they are 

 all broken away ; but the form of the fragment itself, tapering 

 at one extremity and suddenly expanding at the other, as like- 

 wise the columnar structure of the bone for the support of the 

 teeth, prove this to be an imperfect maxillary of Rhizodopsis 

 sauroides. These peculiar pillars of bone supporting the teeth 

 are very characteristic of the jaw-bones of this fish ; but in the 

 pramaxilla they are most developed. Some of our specimens 

 (PI. XVI. fig. 5) are precisely similar to that figiured as 

 Characodus^ the teeth having been all broken away, with the 

 exception of three or four. The display of this curious struc- 

 ture depends much on the plane of the sections ; it is possible 

 to cut it nearly all away, leaving merely the external layer of 

 bone on one side ; and it is never developed to the same 

 extent in the prasmaxilla and mandible. 



The pra3maxilla is the basis of the genus Gastrodus (pis. 14 

 & 15) the supposed Batrachian, as is evinced by the shape of 

 the fragment, the size, form, character, and disposition of the 

 teeth ; nor is there any important difference in the minute 

 structure of the teeth in this so-called genus. According to 

 Prof. Owen's measurements, the dentinal tubules in Dittodus 

 parallelus have a diameter of , „ j, qq of an inch, in Characodus 

 I a o of an inch, and in Gastrodus To-g-o-o j while in Rhizo- 

 dopsis we have ascertained that they are likewise about , oooo 

 of an inch in diameter. The teeth of the so-called Gastrodus 

 are certainly represented to be without enamel ; but we have 

 seen that it is frequently absent in Rhizodojjsis ; and many of 

 the teeth, as exhibited in the figure, are cut diagonally short, 

 so that their form and proportions are destroyed. The ap- 

 pearance thus presented is very common in sections of minute 

 jaws, and, unless clearly understood, may readily lead to error. 

 The diagonal section of a quill illustrates this very well. 



The bone-cells of the jaw of Rhizodojjsis are quite as Ba- 

 trachian as are those figured of the pseudo- Gastrodus ; and so 

 are those of Megcdichthys and many other sauroidal fishes. 



There is, then, no evidence in the paper referred to of a 

 minute air-breathing Batrachian of the age of the lower seams 

 of the Northumberland coal-field, the so-called genus Gastrodus 

 being resolvable into Rhizodopsis sauroides, a Ganoid fish. 



Ctenodus cristatus. 

 Since the publication of the paper on Ctenodus *, the matrix 

 * ;\jiu. & Mag. Nat. Hist. Fob. 1868. 



