Dr. L. Himakof- — U. Devonian Arthrodira from O/iio. 123 



Gain. Loss on each Constituent. 



The loss on the total silica is not as high as that shown on the 

 combined silica, as part of the latter has been changed to quartz, the 

 loss on the total silica being 46-16 parts out of 51'09, equivalent to 

 90-3 per cent, of that substance. Similarly, the loss on the ferrous 

 iron is to some extent set off by a gain in ferric iron, the actual loss 

 of the oxides of iron being T74 parts out of 10*29, or 16'9 per cent. 

 The loss of all constituents of the original diabase amounts to 68*2 per 

 cent., and this loss is accompanied by a gain of 7*40 per cent, of 

 water and of -59 of 1 per cent, of oxygen, so that the decomposition 

 of 100 parts of diabase has resulted in the formation of 39 9 parts of 

 a bauxitic or aluminous laterite. 



The position and mode of the exposure of the diabase, a small boss 

 intrusive in a coarse-textured hornblende-granitite-gneiss, indicates 

 that the lateritization is of comparatively recent origin ; and it is 

 noticeable that the action is simple, consisting only of such changes 

 as could be produced by the long-continued action of percolating 

 water. The action has taken place above the water-table, and the 

 removal of combined silica from the bases present has been very 

 complete ; as far as analytical determinations show, none of it having 

 been retained in combination as kaolin. The outer crust differs from 

 the inner one by its lesser contents of the iron oxides and their lower 

 state of hydration, and to a slight extent by the lower hydration of 

 the aluminium hydrate. If by continued action of water containing 

 readily oxidizable organic matters the iron oxide, other than that 

 present in ilmenite, of the outer crust were removed, a laterite 

 containing over 70 per cent, of aluminium hydrate would result. 



V. — Notes on some Upper Devonian Arthrodira from Ohio, U.S.A., 

 in the British Museum (Natural History). 



By L. Hussakof, Ph.D., Associate Curator of Fossil Fishes in the American 

 Museum of Natural History. 



(PLATE VIII AND THEEE TEXT-FIGUEES.) 



SOME years ago the British Museum acquired from Dr. William 

 Clark a collection of the remarkable Arthrodira found by him 

 in the Upper Devonian shales of Ohio. In this material there are 

 several specimens which were described as new before being brought 

 to Europe, but concerning which there has been more or less doubt 

 among students of the Arthrodira. Hitherto, however, it has been 



