The main thesis of this paper is to show that the Ludlow Bone-bed forms the true base of the Devonian System. This is urged on several grounds.
(a) Priority. It was the limit originally used by Murchison. Although subsequently altered by him, one finds if one attempts to use his later limit:—
(i) That the Old Red Sandstone was not re-defined accordingly.
(ii) That Dumont's Rhénan (Devonian) System, defined in 1845, has priority over Murchison's later definition (1848).
(iii) That the later limit adopted by Murchison does not form a true horizon and has been very differently interpreted by later writers.
(i) There is a marked faunal change at the Ludlow Bone-bed. The Bone-bed marks the first appearance of fossil fishes, and thus the Devonian marks the dawn of an age of vertebrates. There is only one old record of a pre-Devonian fish in the British Isles; one doubtful one in Gotland, and an anomalous one in the Ordovician of America. The Shropshire succession forms a standard of comparison for all areas of the Old Red Sandstone type of deposit.
(ii) The limit adopted after a detailed study by other writers of an uninterrupted marine series is on the same horizon. The succession at Liévin (Northern France) forms a standard for comparison of all areas of the Devonian (marine) type of deposit.
(c) Stratigraphical. In England the Ludlow Bone-bed marks also a change in physical conditions, more noticeable elsewhere as an unconformity. Physical breaks or unconformities on the same horizon occur in Scotland, Belgium, Brittany (slight), Norway, Sweden, and Spitsbergen.
(d) Practical Considerations. The Downtonian rocks, i.e. the strata from the Ludlow Bone-bed horizon upwards, form a stratigraphical and palæontological entity, incapable of broad or even of rough separation below the Dittonian. The Ludlow Bone-bed, on the other hand, forms an horizon which can be recognized and mapped all over North-Western Europe. In England there appears to be a slight faunal break between the Downtonian and Dittonian, but this is less marked elsewhere.