REMAINS OF REPTILES AND FISHES. 85 



comment on the value of the various genera and species recently 

 proposed by Prof. Owen in his paper on the "Dental Characters 

 of Genera and Species, chiefly of Fishes, from the Low-Main 

 Seam and Shales of Coal, Northumberland.'"'' It has become 

 necessary to do this, as the anticipated beneficial results from 

 the former " Criticism" of the "Abstract" of the paper as read 

 have not been realized,! though the influence of this criticism is 

 distinctly traceable in the text of the published paper, as well as 

 in the appended foot-notes. 



The first genus we have to refer to is that named Mioganodus 

 (pi. 8), which is founded on the section of a tooth that in no 

 respect diff'ers from that of the so-called Rhizodus lanceiformis, 

 Newberry. We have shown in the former part of this communi- 

 cation that this reputed fish is most probably a Labyrinthodont 

 amphibian ; but be this as it may, we have teeth of this species 

 attached to the dentary bone exactly similar in contour to, and 

 not larger than, the figure of the tooth of this so-called new 

 genus : and when a longitudinal section of these teeth is exam- 

 ined under the microscope, there is no perceptible diff'erence in 

 the minute structure from that of the tooth of Mioganodus ; even 

 the concentric layers of dentine, which are considered character- 

 istic, are equally well marked. Certainly, when the tooth of R. 

 lanceiformis is perfect, the base exhibits the Labyrinthodont in- 

 folding of the peripheral wall of dentine ; but when the tooth is 

 found detached (and that figured by Prof. Owen was so found), 

 the basal portion is rarely if ever present : and then the dentinal 

 walls are observed to thin out from the interior and to terminate 

 below, when seen in section, in sharp wedge-shaped points, just 

 as they are represented in the figure of Mioganodus laniarius. 

 The tooth, then, on which this genus is founded is merely the 

 upper portion or crown of a tooth of the so-called Rhizodus Ian- 

 ceiformis. 



Rhizodopsis saukoides, sp., Williamson. 

 Several specimens of the elegant fish upon which Professor 



♦"Transactions" Odontological Society, 18fi7. 

 t " Geological Magazine," Vol. IV., pp. o23 and 378. 



