90 MESSES. HANCOCK AND ATTHEY ON 



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 as- 

 sume the regular tubular appearance, in accordance with the in- 

 creased 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 pos- 

 terior extremity broken off and turned forwards ; and all the 

 ianiary teeth, with the exception of the anterior one, are lost, as 

 we have already seen is frequently the case in the mandibles of 

 Rhizodopsi$. The size, form, and surface sculpture 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 PJdzodojms. Ganolodus 

 Craggesii, Owen, will therefore have to give place to Rhizodojms 

 sauroides, sp., Williamson. 



Ganolodus sicula. (pi. 7) is very intimately related to a very 

 diiferent 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 

 a Ianiary 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 Megalichthys 

 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 plicae, as may be seen on referring 

 to fig. 1 b, pi. 7. Prof. Owen calls these fragments "part of the 

 parietal dentine." Were this strictly correct, the calcigerous 

 tubules would be seen cut across, producing 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 demon- 

 stration that these fragments are portions of the basal plicae. To 



