from the Shales of the Northumberland Coal-field. 353 
inner layer of dentine. A similar substance occurs in many 
of our sections, exhibiting the general appearance and dotted 
structure given to it in Prof. Owen’s figure; and this is 
undoubtedly the inner film of dentine; and the dots are the 
orifices of the calcigerous tubules. When the film is a little 
thicker, the dots become elongated; and in other specimens 
they gradually assume the regular tubular appearance, in ac- 
cordance with the increased thickness of the section. 
Ganolodus Craggesit is founded on a mandibular bone of 
the same fish. This fragment is a little distorted, and has the 
posterior extremity broken off and turned forwards ; and all 
the laniary teeth, with the exception of the anterior one, are 
lost, as we have already seen is frequently the case in the 
mandibles of Rhizodopsis. The size, form, and surface-sculp- 
ture of the bone, which latter is well represented in the wood- 
cut, as well as the character, size, and arrangement of the 
teeth, all prove this. 
There is no difference whatever between this mandibular 
ramus and several that are now before us of Rhizodopsis. 
Ganolodus Craggesti, Owen, will therefore have to give place 
to Rhizodopsis sauroides, sp., Williamson. 
Ganolodus sicula (pl. 7) is very intimately related to a very 
different fish. The tooth on which this species is sought to 
be established is perhaps the commonest in the shales of the 
Low-Main seam ; it belongs to Megalichthys, and is apparently 
- alaniary tooth of a young specimen. There is not the slightest 
perceptible difference in the form and structure of the tooth, 
as represented in the figure of this so-called species, and the 
form and structure of the numerous sections of teeth of Megal- 
ichthys which we happen to possess. ‘That the specimen 
figured was grooved and plicated at the base, like the tooth of 
this fish, is proved by the remnants of the plice, as may be 
seen on referring to fig.10, pl. 7. Prof. Owen calls these 
fragments ‘part of the parietal dentine.” Were this strictly 
correct, the calcigerous tubules would be seen cut across, pro- 
ducing the appearance of dots more or less elongated, as is 
well represented by Mr. T. West in pl. 14. fig. 4 (Gastrodus). 
On the contrary, the tubules in the fragments alluded to are 
all exhibited lengthwise, as they are in the cut edge of the 
peripheral dentine—proving to demonstration that these frag- 
ments are portions of the basal plicee. To be satisfied of this, 
it is only necessary to examine a longitudinal section of the 
tooth of Megalichthys or any other tooth with a plicated base. 
The variety G. undatus (pl. 7. fig. 7) is most assuredly the 
tooth of Strepsodus sauroides, Huxley: the double bend of the 
apex and general proportions of the crown put this beyond 
doubt. 
