102 Proceedings of the Boycd Physical Society. 



cate along the sides of the spine ; but in front, where the 

 tuberculation appears, they hecome coarse and curve a little 

 foriuard, 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 

 curvature 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 nohilis may easily be distinguished from both 

 G. formosus and G. tuhercnlatus (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. tuherculatus, 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. tuherculatus show invariably, so far 

 as I have observed, a strongly-marked lateral curvature ; in 

 G. nohilis, 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. ohliquus of M'Coy, and it is certainly not G. den- 

 ticulatus of Davies. Nor can it be shown to be identifiable 

 with any of the North American species named by Professor 

 Newberry and Dr Dawson. 



There only remains the G. alnwicensis of Agassiz, which 

 is recorded from 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 



