x PREFACE. 
known localities, would have occupied fewer pages, and 
would have been thrown off with, perhaps, less regard to 
minute detail than to pictorial effect. May I crave, while ad- 
dressing myself, now to the one class, and now to the other, 
the alternate forbearance of each ? 
Such is the state of progression in geological science, that 
the geologist who stands still for but a very little, must be 
content to find himself left behind. Nay,so rapid is the prog- 
ress, that scarce a geological work passes through the press 
in which some of the statements of the earlier pages have 
not to be modified, restricted, or extended in the concluding 
ones. The present volume shares, in this respect, in what 
seems the common lot. In describing the Coccosteus, the 
reader will find it stated that the creature, unlike its contem- 
porary the Pterichthys, was unfurnished with arms. Ere 
arriving ‘at such a conclusion, I had carefully examined at 
least a hundred different Coccostei ; but the positive evidence 
of one specimen outweighs the negative evidence of a hun- 
dred; and I have just learned from a friend in the north, 
(Mr. Patrick Duff, of Elgin,) that a Coccosteus lately found a‘ 
Lethen-bar, and now in the possession of Lady Gordon 
Cumming, of Altyre, is furnished with what seem uncouth, 
paddle-shaped arms, that project from the head.* All that I 
* As these paddle-shaped arms have not been introduced by Agas- 
3iz into his restoration of the Coccosteus, their existence, at least as 
arms, must still be regarded as problematical. There can be no doubt, 
