Genus Gyracanthus, Agassiz. 45 
common occurrence in the ironstone worked at Borough Lee, 
near Edinburgh, belonging to the Middle Carboniferous Lime- 
stone series of Central Scotland; and I have also seen a frag- 
ment from a similar horizon at Cowdenbeath, in Fifeshire. At 
first I confounded them with G. tuberculatus, Ag., but the 
accession of more extensive material, along with a closer in- 
vestigation of the subject, soon convinced me of their specific 
distinctness. 
Gyracanthus nobilis attains a large size. One spine in my 
own collection, wanting a small portion of the base, but having 
its extreme point preserved, measures 21 inches ; had it been 
entire its length could not have been less than 2 feet. Another, 
wanting the point, must have been about the same size; and 
fragments are not uncommon which indicate still greater 
dimensions. The general form is elongated and slender, the 
breadth increasing more rapidly towards the base in adult 
specimens. ‘Théy are very variable in respect of curvature : 
in some both antero-posterior and lateral curves are well 
marked ; in others the lateral bend is only slight or hardly 
perceptible ; and I have one which appears almost perfectly 
straight in both directions. Every one of them, without ex- 
ception, is nevertheless asymmetrical as regards those special 
points of configuration upon which I have dwelt in connexion 
with G. tuberculatus, and, as in that species, they may be 
arranged in pairs. 
In the form of the non-sculptured inserted part, with its pos- 
terior sulcus, and in the general configuration of the spine as 
seen in transverse sections, G. nobilis closely resembles CG. 
tuberculatus. The posterior marginal keel is in its distal 
portion strongly denticulated ; in one specimen the denticles 
may be traced, from the point, a distance of 10 inches in the 
direction of the base. ‘The posterior groove varies much in 
its degree of sharpness ; in some it is very shallow and slightly 
marked till towards the point, while in others it is very well 
defined along its whole extent. In adult specimens continu- 
ations of the gyrating ridges usually encroach upon it at its 
commencement; but the salient point in this species lies in 
the disposition and mode of tuberculation of these ridges. 
At the proximal end of the spine, in adult examples, they 
are disposed much as in G. tuberculatus, meet each other 
anteriorly at much the same angle, and are closely tubercu- 
lated along their whole extent. But near the closure of the 
sulcus this close tuberculation becomes limited to the anterior 
aspect, each ridge as it arises and advances forward showing 
first a comparatively distant tuberculation, then a smooth 
space (sometimes very minutely crenulated) on the side of the 
