94 Proceedings of the Royal Physical Society. 



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,^ 

 pointed out that the groove is obviously equivalent to the pos- 

 terior flattened area in such median spines as CtenacantMis, but 

 here turned awry and looking to one side, whUe 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 Qyr acanthus. 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 vari- 

 able distance from the closure of the sulcus, and only marked 

 by delicate longitudinal striae, while in one I find it devoid 

 of tubercles along its whole extent. In some too, before the 

 truncation of the apex occurs, the gyrating ridges tend to lose 

 their close tuberculation, at least posteriorly, and to become 

 only distantly nodulose or even quite plain. 



Putting the wearing of the tips altogether aside as a secon- 

 dary question, the striking want of bilateral symmetry in 

 these spines, together with their occurrence in " rights and 



1 Geol. Mag., dec. ii., vol. ix. (1882), p. 542. 



