48 On the Genus Gyracanthus, Agassiz. 
variable; some are indeed in that direction nearly quite 
straight. 
The first salient point which strikes the eye is the great 
size of the inserted or non-sculptured portion, which is not 
only broader and more expanded, but extends further beyond 
the sculptured part proximally than in any other species. ‘The 
anterior middle line on which the gyrating ridges meet does 
not cut equally the very acute angle formed proximally by 
the sculptured part; but in this case the larger division is 
found on the subgibbous side, this being due to the encroach- 
ment of the non-sculptured part on the gibbous side. It is 
next to be noticed that the shaft of the spine after the closure 
of the sulcus is more cylindrical than in other species; still 
the want of bilateral symmetry is very obvious, and a gibbous 
and subgibbous side may be distinguished. ‘The posterior 
groove is sometimes not apparent for some distance after the 
closure of the sulcus, or, though indicated, it may be filled 
with tubercles ; sooner or later it becomes well marked, and 
the lip on the subgibbous side becomes more prominent than 
the other, but does not form so marked a feature in the con- 
figuration of the spine as the corresponding posterior marginal 
keel in such species as G. tuberculatus and nobilis ; it is in fact 
only towards the extremity, that the spine takes on a keeled 
appearance. A well-marked row of recurved denticles occurs 
along the aforesaid lip or ridge of the posterior groove on the 
subgibbous side, and on that of the opposite side denticles are 
also seen in some examples. ‘The last remarkable feature in 
this species is the slight obliquity of the gyrating ridges, which 
meet each other on the front of the spine at angles greater 
than right angles almost as far as the very apex. These 
ridges are also rather less oblique on the subgibbous than on 
the gibbous side ; on the former they are in fact sometimes 
nearly transverse; a certain amount of sigmoidal curvature 
is assumed after the middle of the spine, the anterior extre- 
mities of the ridge turning slightly towards the apex, their 
posterior extremities towards the base. Only towards the apex 
have the ridges any marked obliquity in their middle portions, 
and there they often also become wavy. ‘The gyrating ridges 
are closely tuberculated over their whole extent, except to- 
wards the apex, where the tuberculation tends to become 
irregular. ‘The amount of apical wearing is very variable. 
