426 RAMBLES OF A GEOLOGIST. 



and higher layers, anxious to ascertain at what distance over 

 the base the more ancient organisms of the system first ap- 

 pear, and what their character and kind. And little more 

 than a hundred yards over the granite, and somewhat less 

 than a hundred feet over the upper stratum of the Great 

 Conglomerate, I found what I sought, a well-marked bone, 

 perhaps the oldest vertebrate remain yet discovered in Ork- 

 ney, embedded in a light grayish-coloured layer of hard flag. 

 What, asks the reader, was the character of the ancient 

 denizen of the Palaeozoic basin of which it had formed a part ? 

 Was it a large or small fish, or of a high or low order ? Not 

 certainly of a low order, and by no means of a small size. 

 The organism, in the rock was a specimen of that curious 

 nail-shaped bone of the Asterolepis which occurs as a central 

 ridge in the single plate that occupies in this genus the wide 

 curve of the under jaw ; and as it was fully five inches in 

 length from head to point, the plate to which it belonged 

 must have measured at least ten inches across, and the fron- 

 tal occipital buckler with which it was associated, one foot 

 two inches in length (not including the three accessory plates 

 at the nape), by ten inches in breadth. And if built, as it 

 probably was, in the same massy proportions as its brother 

 Co3lacanths the Holoptychius or Glyptolepis, the individual 

 to which the nail-shaped bone belonged must have been, judg- 

 ing from the size of the corresponding parts in these ichthyo- 

 lites, at least twice as large an animal as the splendid Clash- 

 bennie Holoptychius of the Upper Old Red, now in the Bri- 

 tish Museum. The bulkiest ichthyolites yet found in any of 

 the divisions of the Old Red system are of the genus Astero- 

 lepis ; and to this genus, and to evidently an individual of 

 no inconsiderable size, this oldest of the organisms of Orkney 

 belonged. I was so interested in the fact, that before ulti- 

 mately leaving this part of the country I brought Dr Gar- 

 son, Stromness, and Mr William Watt, jun., Skaill, both 



