1879.] of life forms to breaks of continuity in the strata. 257 



for the head shield of Pteraspis> for instance, was a thing easily- 

 preserved and recognised. 



Quite at the top of the Ludlow Kocks we have a bone bed 

 full of remains of Onchus and Thelodus, which from their likeness 

 to sharks we may suppose to have had a high brain organization. 

 There is a cansiderable difference between this group and that which 

 occurs in the Upper Old Red or base of the Carboniferous series, 

 but a long time, measured by long deposition and enormous inter- 

 mediate denudation, has elapsed between the two periods. When 

 we find the Cambrian fish, which I fully expect we shall, we shall 

 see that they too are very different from those of the Ludlow Rocks. 



In the succeeding Devonian and Carboniferous periods they 

 abound. But the doubts as to the grouping of the Old Red and 

 Devonian, and the limited distribution and range of this group, 

 renders it less suitable for our present purpose. As far as the 

 evidence does go it quite confirms the inference we have already 

 arrived at : that lapse of time, whether measured by deposition or 

 denudation, generally is accompanied by the introduction of new 

 or modified forms of life. 



Whatever may be said of the extension of the classification 

 here adopted, as far as I am inclined to apply it, it is clear that we 

 may consider a great part of the Devonian as a basement series to 

 the Carboniferous ; and bearing this in mind we will take as an 

 example the genus Producta, and trace it back to see whether it 

 comes in after an unconformity. We do, it is true, find it low 

 down in the Mountain limestone, even when that rests on the 

 upturned edges of the Silurian, as in the Craven area in York- 

 shire; but unless we recognise it as represented in the Productella 

 of the Devonian or the Chonetes and Leptaena of the Silurian 

 and Cambrian, it is not found in earlier beds. 



When the Carboniferous sea basin, in which the coral and 

 the encrinite grew and the Productidae thrived in thousands, was 

 being silted up or raised, then the animals of the clear sea or their 

 spawn migrated to less unfavourable areas, and appeared as new 

 forms in the midst of locally continuous deposits. As to what 

 were the forms from which they were originally modified we have 

 only rarely a suggestion. For instance, the Cephalopoda are, in 

 the Devonian, modified in the position of the siphuncle and other 

 characters, so as to foreshadow the two great groups of the next 

 period ; the Goniatites, with its dorsal siphuncle, leading up to 

 the Ammonites and the Clymenia to the genus Nautilus. What 

 variation in character was produced in the process of acclimatisa- 

 tion in the new. home of each part of their race we seldom can 

 examine closely enough to enquire, but we may hope by and bye 

 to get from this line of enquiry also some evidence bearing upon 

 the great geographical changes our earth has undergone. 



