Length of a typical spine 1⅞ inches; greatest diameter, inch. The spine is thickest about half an inch from its proximal extremity, towards which it slightly contracts, while distally it tapers to a fine point; its contour is at first straight, then becoming gently curved backwards towards the apex. Transverse section at first round, but near the middle of the spine a flattened atea commences, running along the posterior margin, and as this is followed onwards, a delicate median ridge appears, and is continued to the apex. Along the boundary, on each side, of this posterior area, a right and left row of denticles is seen extending from the apex to three-quarters of an inch from the base: they thus occupy a greater extent of the length of the spine than the area in question. These denticles are about 38 on each side, opposite, closely set, minute, their bases compressed and proportionally elongated, their sharp, conical points directed obliquely downwards. Surface of spine striated proximally by delicate longitudinal grooves, which become gradually lost towards the perfectly smooth apex. As usual, a rounded central cavity commences at the base and extends towards the apex.