46 Dr. R. H. Traquair on the 
spine, and finally becoming thick and coarsely tuberculated 
as it turns round to the front. Where this feature of the 
ridges commences they also become excessively oblique and very 
delicate, and in some specimens they also occasionally bifur- 
cate along the sides of the spine; but in front, where the 
tuberculation appears, they become coarse and curve a little 
JSorward, so as to become less oblique, and in many cases they 
turn slightly again towards the point just before meeting those 
of the opposite side. ‘Towards the extremity the ridges become 
entirely smooth on the sides of the spine, their slight curva- 
ture also ceases, and the tuberculation of the anterior aspect 
gives way to simple undulation. ‘The point, even where it is 
not positively truncated by attrition, looks smooth and 
rubbed. 
Gyracanthus nobilis may easily be distinguished from both 
G. formosus and G. tuberculatus (probably only varieties of one 
common species) by the direction of the gyrating ridges. In 
the latter forms these ridges are disposed in a pretty straight 
and parallel fashion over the sides of the spine, although they 
do increase in obliquity towards the apex. Here, however, 
their excessive obliquity and delicacy along the sides, after the 
closure of the sulcus, give the sculpture a peculiar aspect 
which cannot be mistaken. The tuberculation of the ridges 
is in general coarser than in G. tuberculatus, and, in the 
latter, it is only pretty well towards the apex that the ridges 
tend proximately to become plain, or only distantly nodulose. 
Of course, as regards the disposition of tuberculation, this new 
species differs still more from G. formosus, in which the ridges, 
from the very base, tend to be plain in front. 
Adult specimens of G, tuberculatus show invariably, so far 
as I have observed, a strongly-marked lateral curvature ; in 
G. nobilis, as we have seen, its presence and amount is very 
variable. 
The course of the ridges, the disposition of the tubercula- 
tion, and the form of the transverse section equally distinguish 
it from G. obliquus of M‘Coy, and it is certainly not G. den- 
ticulatus of Davis. Nor can it be shown to be identifiable 
with any of the North-American species named by Prof. 
Newberry and Dr. Dawson. 
There only remains the G. alnwicensis of Agassiz, which 
is recorded trom a somewhat similar horizon, viz. the Car- 
boniferous Limestone series of Alnwick, in Northumberland. 
This is very briefly mentioned by Agassiz as being slender in 
form, with very oblique and entirely smooth or non-tubereu- 
lated ridges, which ridges also bifurcate, and even trifureate, 
in a very remarkable manner, as shown in the figure. If this, 
