Genus Gyracauthus, Agassiz. 39 
at from the front, displays a well-marked lateral curvature or 
bend, which enables us to distinguish a convex and a concave 
side. It will also be observed that the lateral surface is more 
gibbous or rounded on the convex aspect of the spine, flatter 
on the opposite, so that for purposes of description we may 
distinguish the two sides as ‘ gibbous’”’ and ‘ subgibbous ”’ re- 
spectively. Still regarding it from the front, it will be seen 
that the sculptured surface ends proximally in an acute angle ; 
but the apparent middle line on which the tuberculated or 
“ ovrating ” ridges meet does not bisect this angle, but divides 
it so that the sculptured part is larger on the gibbous side. 
Now, turning the spine over so as to look at it from behind, 
we observe that the longitudinal cleft or sulcus leading into 
the central cavity is not in the middle of the non-sculptured 
inserted part, but is placed more towards the subgibbous side, 
so that we have here from the very beginning a marked 
deviation from bilateral symmetry, one side, the convex or 
gibbous one, being larger than the other. We next observe 
that, from the distal closure of the sulcus, the lip on its sub- 
gibbous side is continued onwards towards the apex as a 
blunt keel or margin, having on the gibbous side a shallow 
longitudinal depression or groove. Thus the spine has now 
become keeled or marginated posteriorly, and from this mar- 
gin round to the line of convergence of the gyrating ridges 
in front the surface on the subgibbous side is narrower and 
flatter, while on the opposite or gibbous aspect it is more ex- 
tensive, more rounded, and provided with the aforesaid longi- 
tudinal groove. I have already, on a previous occasion*, 
ointed out that the groove is obviously equivalent to the pos- 
terior flattened area in such median spines as Ctenacanthus, but 
here turned awry and looking to one side, while the posterior 
marginal ridge represents one of the denticulated margins in the 
last-named genus; the other is to be looked for in the opposite 
or feebly-marked edge of the groove on the gibbous side in 
Gyracanthus. The sculptured or gyrating ridges are on the 
whole pretty straight and parallel in their course, though they 
show a slight tendency to a sigmoidal direction, curving a 
little towards the apex in front, towards the base behind, as 
well as increasing progressively in obliquity from the base 
onwards. They are closely tuberculated along their whole 
extent, and are continued as lines of tubercles over the lips of 
the posterior groove, in the bottom of which they converge 
and meet. In the above described specimen the groove is 
filled with tubercles as far as the spine reaches ; but in others 
the groove becomes bare of tubercles at a variable distance 
* Geol. Mag. dec, ii, vol. ix. (1882), p. 542. 
