from the Shales of the Northumberland Coal-field. 353 



■inner layer of dentine. A similar substance occurs in many 

 of our sections, exhibiting- tlie 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 Craggesii 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 Uhizodopsis. 

 Ganolodus Craggesii^ Owen, will therefore have to give place 

 to FiMzodopsis sauroides^ sp., Williamson. 



Ganolodus sicula (pi. 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 Megaltchthys^ and is apparently 

 a laniary 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 plicaj, as may be 

 seen on referring to fig. 1 J, pi. 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 pi. 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 plicte. 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 (pi. 7. fig. 7) is most assuredly the 

 tooth of Strepsodus sauroides, Huxley : the double bend of the 

 apex and general proportions of the croAvn put this beyond 

 doubt. 



