The spine is 40 mm. in length, by 5 mm. in width at the junction _L of the inserted and exserted portions; from this point the spine gradually narrows to a moderately fine point at the apex. The posterior margin is nearly straight, the anterior one gently curved, the curve being more marked distally. The lateral surfaces meet in front at a moderately acute angle, the spine being much laterally compressed. The posterior surface is concave, and each margin is armed with the usual well-marked recurved denticles. The sides of the exserted portion are ornamented with a longitudinal series of ganoine-coated, tuberculated ridges.