Correspondence — Mr. J. Clifton Ward. 581 



probable, by the fact that Professor Kner has found Bijylodi placed 

 in rows in the jaws of a Fish which he designates Xenacanthus. The 

 Professor asserts that '* in the lateral parts of the jaws they are ar- 

 ranged in 28-29 rows, of 6-8 in each transversely, and on the outer 

 maxillary they form 4 rows of G-8 in each." If the observations of 

 Professor Kner are perfectly reliable, the question is settled, and the 

 Diplodi are teeth, but the evidence, so far as regards our Coal-measure 

 specimens, points rather in the direction of their being tubercles, in- 

 asmuch as they are often found in large diffused patches, without 

 any appearance of arrangement, and without the slightest indication 

 of corresponding jaws in these localities. This evidence, however, is 

 negative, and no merely negative evidence is of value in the presence 

 of that which is positive. The only questions therefore to be deter- 

 mined are : Are the specimens described by Professor Kner gene- 

 rically the same as those found in our Coal-measures, and is the ar- 

 rangement he describes so satisfactory as to be conclusive ? It is 

 acknowledged that many of the Professor's specimens are indistinct 

 and dubious, and the question may without offence be asked, '' Was 

 the specimen ja^v and teeth, or supposed teeth dubious or clear?'* 

 The writer of the review of the work on Xenacanthus in the Geol. 

 Mag. for August, observes : " Our author thinks that the strata in 

 England and North America, containing the teeth of Biplodiis and 

 spines of Pleuracanthus, ought perhaps, on closer examination, also to 

 be referred to the Permian system, and not to the Carboniferous. 

 In this supposition Professor Kner is manifestly wrong, as Biplodi 

 are among the commonest fossils of our Northumberland Coal-mea- 

 sures. On the table before us there are specimens of Diplodi, and 

 some large slabs in which large bones are present and numerous 

 Biplodi scattered about. There is also a mass of comminuted bones, 

 in the midst of which Diplodi lie buried in considerable number. 



COI^I?,:E:]s:po^s^IDE3^CE, 



INTERNAL FLUIDITY OF THE EARTH. 



Sir, — In this month's Geological Magazine no less than three 

 articles occur treating of the internal constitution of the earth, and 

 the manner of ''Formation of Mountain Chains." 



Mr. Fisher explains the "Elevation of Mountain Chains" by an 

 outer crust becoming unsupported through the contraction by cooling 

 of the inner parts of the earth, the intermediate layer then (by 

 diminution of pressure) becoming the most fluid portion. 



M. Delaunay says that, if the motion be slow, the rotation of a 

 solid crust would be accompanied by that of the fluid interior, and 

 therefore that no idea of the thickness of this outer crust can be 

 formed from the phenomena of precession and nutation. 



Mr. Shaler argues that, "if the effect of pressure in promoting 

 solidification at the earth's centre were greater than the effect of heat 

 in resisting solidification, then the mass would congeal first at the 

 centre, and solidification extend thence towards the circumference : " 



