446 Prof. W. C. Williamson. On the Organization [Mar. 27, 



filled up with volcanic ash. These steins failed to display a single 

 feature, justifying the conclnsion that they were Sigillarian. 



In two of them the central cavity, instead of being filled with ash, 

 was filled with miscellaneous heaps of vegetable matter, amongst 

 which were large fragments of the vascular axes of various plants, 

 such as Lepidodendra and Stigmarice, but in one of the largest stems 

 were five or six decorticated vascular cylinders of Diploxyloid stems, 

 of the largest size, and which, though arrauged parallel to the 

 long axis of the cylinder which enclosed them, obviously did not 

 belong to them, but had been floated in from without. The sup- 

 position that these had been young stems that had grown within 

 the hollow protecting cylinders, from spores, accidentally intro- 

 duced, is wholly untenable, since each one of these several vascular 

 axes has been the centre of a stem fully as large as that within 

 which we found them aggregated. Of course, these Diploxyloid 

 vascular axes had the organization which Brongniart and the younger 

 school of French botanists which still upholds his views on this 

 point, believe to be characteristic of true Sigillariaa — a conclusion from 

 which I have long dissented. 



The only fragment we found, that threw any light upon the character 

 of the leaf-scars that had indented the surfaces of these fully grown 

 stems, was a well-defined example of the Lepidodendroid type. 



We directed careful attention to the nature of the smaller fragments 

 of branches and foliage which abounded in the volcanic ash with 

 which the large stems were overlaid. These consisted of Lepido- 

 dendroid branches and twigs of all sizes and ages, and no doubt was 

 left upon mv mind that they were really the disjecta membra of the 

 stems around which they were so profusely scattered. The only fruits 

 that have been obtained from the same locality are Lepidostrobi, most 

 of which contain macros pores and microspores. Unless we are prepared 

 to believe that this Arran deposit contained, on the one hand, numerous 

 stems without branches, and, on the other, yet more numerous branches 

 without stems, we must recognise in these specimens the comple- 

 mentary elements of a grove of Lepidodendroid trees. 



One specimen found is a very important one. It has a mean 

 diameter of six inches, and is either a small stem or a very large 

 branch. Internally it exhibits the same structure as all the smaller 

 Lepidodendroid branches, except so far as it is modified by size and 

 age. But in addition to its other features, it exhibits a very narrow 

 exogenous ring surrounding the ordinary Lepidodendroid one, thus 

 giving some clue to the size attained by such branches before the 

 internal organization passed from the Lepidodendroid to the Sigillarian 



I have at last succeeded in obtaining the Strobilus, to which the 

 remarkable macrospores and microspores figured in my last memoir 



